Add notification channels and a provider abstraction

Users get a notification_channels table (replacing users.ntfy_topic); pages fan
out to every enabled channel through a notify.Provider registry, with ntfy as
the first provider. Fallbacks are typed {type, address}. Schema edited in place:
existing databases must be recreated.

Claude-Session: https://claude.ai/code/session_016mBLURvJoMuUEr9cB2RpUN

internal/db/migrations/001_schema.sql

@@ -65,7 +65,8 @@ CREATE TABLE escalation_levels (
 CREATE TABLE escalation_policies (
     team_id bigint NOT NULL,
     repeat_count bigint DEFAULT 0 NOT NULL,
-    fallback_topic text DEFAULT ''::text NOT NULL,
+    fallback_type text DEFAULT ''::text NOT NULL,
+    fallback_address text DEFAULT ''::text NOT NULL,
     updated_at bigint DEFAULT (floor(EXTRACT(epoch FROM now())))::bigint NOT NULL,
     CONSTRAINT escalation_policies_repeat_count_check CHECK (((repeat_count >= 0) AND (repeat_count <= 10)))
 );
@@ -159,11 +160,22 @@ CREATE TABLE invites (
     CONSTRAINT invites_role_check CHECK ((role = ANY (ARRAY['owner'::text, 'member'::text])))
 );
 
+CREATE TABLE notification_channels (
+    id bigint GENERATED BY DEFAULT AS IDENTITY NOT NULL,
+    user_id bigint NOT NULL,
+    type text NOT NULL,
+    address text NOT NULL,
+    enabled boolean DEFAULT true NOT NULL,
+    created_at bigint DEFAULT (floor(EXTRACT(epoch FROM now())))::bigint NOT NULL
+);
+
 CREATE TABLE notifications (
     id bigint GENERATED BY DEFAULT AS IDENTITY NOT NULL,
     incident_id bigint NOT NULL,
     user_id bigint,
-    topic text NOT NULL,
+    channel_id bigint,
+    channel_type text NOT NULL,
+    address text NOT NULL,
     kind text NOT NULL,
     created_at bigint NOT NULL,
     send_after bigint NOT NULL,
@@ -268,7 +280,6 @@ CREATE TABLE users (
     username text NOT NULL,
     email text NOT NULL,
     created_at bigint DEFAULT (floor(EXTRACT(epoch FROM now())))::bigint NOT NULL,
-    ntfy_topic text,
     password_hash text,
     is_admin boolean DEFAULT false NOT NULL,
     disabled_at bigint,
@@ -332,6 +343,12 @@ ALTER TABLE invites
 ALTER TABLE invites
     ADD CONSTRAINT invites_token_hash_key UNIQUE (token_hash);
 
+ALTER TABLE notification_channels
+    ADD CONSTRAINT notification_channels_pkey PRIMARY KEY (id);
+
+ALTER TABLE notification_channels
+    ADD CONSTRAINT notification_channels_user_type_address_key UNIQUE (user_id, type, address);
+
 ALTER TABLE notifications
     ADD CONSTRAINT notifications_pkey PRIMARY KEY (id);
 
@@ -452,6 +469,8 @@ CREATE UNIQUE INDEX deadman_switches_team_name_key ON deadman_switches (team_id,
 
 CREATE INDEX invites_team_idx ON invites USING btree (team_id);
 
+CREATE INDEX notification_channels_user_idx ON notification_channels USING btree (user_id);
+
 CREATE INDEX notifications_incident_idx ON notifications USING btree (incident_id, id DESC);
 
 CREATE INDEX notifications_pending_idx ON notifications USING btree (send_after) WHERE (sent_at IS NULL);
@@ -550,6 +569,12 @@ ALTER TABLE invites
 ALTER TABLE invites
     ADD CONSTRAINT invites_team_id_fkey FOREIGN KEY (team_id) REFERENCES teams(id) ON DELETE CASCADE;
 
+ALTER TABLE notification_channels
+    ADD CONSTRAINT notification_channels_user_id_fkey FOREIGN KEY (user_id) REFERENCES users(id) ON DELETE CASCADE;
+
+ALTER TABLE notifications
+    ADD CONSTRAINT notifications_channel_id_fkey FOREIGN KEY (channel_id) REFERENCES notification_channels(id) ON DELETE SET NULL;
+
 ALTER TABLE notifications
     ADD CONSTRAINT notifications_incident_id_fkey FOREIGN KEY (incident_id) REFERENCES incidents(id) ON DELETE CASCADE;