Sign in through an OpenID Connect provider, and from a terminal

terdut can now sign people in through any OIDC provider (written against
Authentik), and let groups at the provider decide who may sign in, which
teams they belong to and whether they administer the install. Password
login keeps working alongside it; TERDUT_PASSWORD_LOGIN=false turns it off,
and is refused at startup unless SSO is configured. With no TERDUT_OIDC_*
setting nothing changes, so every existing install behaves as before.

Identity is (issuer, subject), never email or username: those are mutable
at the provider and a recycled address must not inherit an account. An
existing user is linked by email only when the provider marks it verified,
or TERDUT_OIDC_TRUST_EMAIL is set, which Authentik needs.

Group grants are marked source='oidc' on team_members and users, and the
sync changes only those rows. Hand-made memberships and administrators
are left alone, and the sync bypasses the last-owner and last-admin guards
because the provider is the source of truth for what it grants. Editing
managed access by hand is refused with 409, since the next sign-in would
undo it. The web UI badges it as SSO and disables the controls.

Groups are read only at sign-in, so an SSO session carries a hard ceiling
(sessions.max_expires_at, 12h by default) that sliding never extends.
There is no refresh token, which means API keys of somebody removed at the
provider stay valid until an administrator disables the user. That is
accepted and documented, not fixed.

A client with no browser, the TUI over SSH, signs in with a device code
run by terdut itself (POST /api/oidc/device and /device/token), so the
terminal never talks to the provider and ends up with the ordinary
terdut_session cookie. Only a browser session can approve a code; an API
key cannot. /device?code= sends a signed-out visitor through sign-in and
back, which is what oidc_logins.next is for.

oauth2 is pinned to v0.36.0: v0.37 needs Go 1.26 and the Dockerfile
builds on 1.25.

Migrations 011 and 012 add tables and defaulted columns only.

internal/db/migrations/011_oidc.sql

@@ -0,0 +1,60 @@
+-- Single sign-on through an OpenID Connect provider (Authentik, and anything
+-- else that speaks OIDC).
+--
+-- Four things change, and none of them touches a password user: every new column
+-- has a default that says "this is how it has always worked".
+--
+-- 1. user_identities says which provider account a user is. It is keyed on
+--    (issuer, subject), never on email or username: those are mutable at the
+--    provider, and a recycled address must not inherit somebody's account. A
+--    user can have several identities (a second provider later), and none at all
+--    (a local, password-only user), which is why this is a table and not two
+--    columns on users.
+--
+-- 2. team_members.source and users.admin_source record who granted a role. 'oidc'
+--    rows are owned by the group sync: it adds them when a group grants access
+--    and removes them when it stops, and nothing else may edit them. 'manual' rows
+--    are everything that existed before this migration, and are never touched by
+--    the sync. Without the marker the sync could not tell a membership it created
+--    from one an owner added by hand, and would have to either leave stale access
+--    behind or delete people it had no business deleting.
+--
+-- 3. sessions.max_expires_at is a hard ceiling on a session's life. Ordinary
+--    sessions slide for as long as they are used; a session made by an SSO login
+--    must not, because the login is the only moment the groups are re-read.
+--    Capping the session is what makes "removed from the group in the provider"
+--    take effect within a bounded time. NULL means no ceiling.
+--
+-- 4. oidc_logins holds a login that has been started and not yet finished: the
+--    state, nonce and PKCE verifier the callback must see again. A row rather
+--    than a signed cookie, so it survives a restart and needs no signing key.
+--    Only the hash of the state is stored, like every other token here; the
+--    nonce and verifier are useless without the state that names the row.
+CREATE TABLE user_identities (
+    id            BIGINT GENERATED BY DEFAULT AS IDENTITY PRIMARY KEY,
+    user_id       BIGINT NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    issuer        TEXT   NOT NULL,
+    subject       TEXT   NOT NULL,
+    created_at    BIGINT NOT NULL DEFAULT FLOOR(EXTRACT(EPOCH FROM now()))::bigint,
+    last_login_at BIGINT NOT NULL DEFAULT FLOOR(EXTRACT(EPOCH FROM now()))::bigint,
+    UNIQUE (issuer, subject)
+);
+
+CREATE INDEX user_identities_user_idx ON user_identities (user_id);
+
+ALTER TABLE team_members
+    ADD COLUMN source TEXT NOT NULL DEFAULT 'manual' CHECK (source IN ('manual', 'oidc'));
+
+ALTER TABLE users
+    ADD COLUMN admin_source TEXT NOT NULL DEFAULT 'manual' CHECK (admin_source IN ('manual', 'oidc'));
+
+ALTER TABLE sessions ADD COLUMN max_expires_at BIGINT;
+
+CREATE TABLE oidc_logins (
+    state_hash    TEXT   PRIMARY KEY,
+    nonce         TEXT   NOT NULL,
+    pkce_verifier TEXT   NOT NULL,
+    expires_at    BIGINT NOT NULL
+);
+
+CREATE INDEX oidc_logins_expires_idx ON oidc_logins (expires_at);

internal/db/migrations/012_device_login.sql

@@ -0,0 +1,40 @@
+-- Signing in from a terminal, for clients that cannot open a browser on the
+-- machine they run on (the TUI over SSH is the reason).
+--
+-- The flow is the OAuth device authorization grant, run by terdut itself rather
+-- than the identity provider, so the terminal never talks to the provider and
+-- the server issues its ordinary session at the end:
+--
+--   1. The terminal asks for a login and gets two secrets: a device code it
+--      keeps and polls with, and a short user code it shows the person.
+--   2. The person opens the verification URL on any device, signs in by whatever
+--      means the server offers, sees the user code, and approves it.
+--   3. The terminal's next poll finds the row approved and is given a session.
+--
+-- Only the hash of the device code is stored, like every other token here: the
+-- device code is what earns a session, so a database read must not yield one.
+-- The user code is shown on screens and typed by people, so it is stored as is;
+-- on its own it can only be approved, never redeemed.
+--
+-- user_id is the person who approved. It is empty until then, and the session
+-- is minted at redemption, not at approval: an approval nobody collects must not
+-- leave a live session lying about.
+--
+-- last_polled_at lets the server refuse a client that polls faster than the
+-- interval it was told.
+CREATE TABLE device_logins (
+    device_hash    TEXT   PRIMARY KEY,
+    user_code      TEXT   NOT NULL UNIQUE,
+    status         TEXT   NOT NULL DEFAULT 'pending' CHECK (status IN ('pending', 'approved', 'denied')),
+    user_id        BIGINT REFERENCES users(id) ON DELETE CASCADE,
+    expires_at     BIGINT NOT NULL,
+    last_polled_at BIGINT NOT NULL DEFAULT 0
+);
+
+CREATE INDEX device_logins_expires_idx ON device_logins (expires_at);
+
+-- Where to send the browser once a single sign-on login completes. A person who
+-- opens /device?code=... without a session has to sign in first and then come
+-- back to it, and the same is true of any other deep link. Validated when it is
+-- stored: only a path on this server is ever kept.
+ALTER TABLE oidc_logins ADD COLUMN next TEXT NOT NULL DEFAULT '/';