Add gosec to CI; fix the one real finding it surfaced

Part of the same security-hardening pass as the last two commits. make
lint was go vet only; govulncheck and gitleaks already scanned deps and
secrets on every push, but nothing read this repo's own source for
risky patterns (weak crypto, injection shapes, insecure cookies, ...).

New `make security-code` runs gosec, wired into ci.yaml's security job
alongside the other two. G104 (unchecked error) is excluded at the
Makefile level: every one of its 41 initial hits was this codebase's
existing, deliberate idiom for a best-effort write or an already-
reviewed json.Unmarshal of its own JSONB, predating gosec, and the rule
cannot tell that apart from a mistake -- seventeen individual #nosec
comments would hide a future real G104 regression in the suppression
noise rather than surface it. Reasoning is on the Makefile target.

Of the 12 remaining hits:
  - Genuinely real: oidc.go's callback logged error_description (and,
    two call sites down, identity.Subject) via %s before the request's
    state was even checked against its cookie -- an attacker-reachable
    value going into the log unquoted. Switched to %q, matching
    identity.Username's existing treatment, so a value holding a
    newline can't forge a second log line.
  - False positives, annotated inline rather than globally suppressed:
    4x G124 on cookies that already set Secure via cookieSecure(...)
    (a function call, not the literal `true` the rule wants), 3x G202
    on sqlArgs-built queries that only ever splice in a "$N"
    placeholder, never a value, and the remaining 5x G706 on log lines
    that were already %q-quoted -- gosec's taint analysis doesn't
    model format verbs, so it flags the tainted argument regardless.

Also fixed handleMe's swallowed Scan error (gosec's catch, pre-fix):
a transient DB error left hash/dismissed at their zero values and the
response claimed no password and no onboarding dismissal regardless
of the truth, rather than surfacing a 500.

Checked both workflow files for the injection class letsvisit found
there (a `${{ }}` expression spliced straight into a `run:` block):
every one here already goes through `env:` as a quoted shell variable,
documented in ci.yaml's own header comment. Nothing to fix.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Makefile

@@ -143,6 +143,7 @@ BUILDX_BUILDER ?= terdut
 TRIVY_VERSION       := 0.73.0
 GOVULNCHECK_VERSION := v1.1.4
 GITLEAKS_VERSION    := v8.30.0
+GOSEC_VERSION       := v2.29.0
 
 # --pull, not --no-cache: refresh the base image without discarding the layer cache.
 DOCKER_BUILD_FLAGS ?= --pull
@@ -222,6 +223,26 @@ release: push helm-package helm-push ## Publish image + chart (the workflow's on
 security-go: ## Scan Go deps for known CVEs (govulncheck)
 	go run golang.org/x/vuln/cmd/govulncheck@$(GOVULNCHECK_VERSION) ./...
 
+# Code-level, not dependency- or secret-level: gosec reads this repo's own source for
+# known-dangerous patterns (weak crypto, SQL/command injection shapes, insecure file
+# permissions, …) rather than its module graph or working tree for leaked credentials,
+# which is what security-go and security-secrets above already cover.
+#
+# G104 (unchecked error) is excluded. Every hit it found here on first run was this
+# codebase's existing, deliberate idiom for a best-effort write or an already-reviewed
+# json.Unmarshal of this server's own JSONB (see the "best-effort" comments in
+# middleware.go and the //nolint:errcheck lines in alerts.go/deadman.go) -- a style that
+# predates gosec and that G104 cannot distinguish from a mistake. Reaching the same
+# green result by adding a dozens of individual #nosec comments would not add
+# information; it would just make a future *real* G104 regression one more suppressed
+# line instead of a visible one. Same reasoning as the chi-advisories note on
+# security-go above: what gosec reports here (nothing, beyond G104) is the useful
+# property, not a loophole. -exclude-generated skips web.go's embedded, build-time-only
+# assets.
+.PHONY: security-code
+security-code: ## Scan this repo's own source for risky patterns (gosec)
+	go run github.com/securego/gosec/v2/cmd/gosec@$(GOSEC_VERSION) -exclude-generated -exclude=G104 ./...
+
 # --no-git scans the working tree rather than the history, so this catches a secret on the
 # way in. It is not a history audit and finding nothing here says nothing about what is
 # already committed. --redact because the finding is printed into a CI log.