Adopt pressly/goose for schema migrations

Replaces the previous hand-rolled MigrateWithLock + GORM AutoMigrate path,
which had two compounding problems:
- AutoMigrate ran on every pod startup (~5 min over the transatlantic
  link) even when no schema changes had landed
- pg_advisory_lock is session-scoped, which silently fails through
  Neon's pgbouncer transaction-mode pooler — turns out this is a
  known and documented limitation that bites golang-migrate too

Goose was chosen over golang-migrate (the other heavyweight) because:
- Goose wraps each migration file in a transaction by default, so a
  failure rolls back cleanly instead of leaving a "dirty" version
  state requiring manual force-reset (golang-migrate's known
  weakness, per its own issue tracker — see #1001 + Atlas's writeup)
- Goose's locking is opt-in. We don't opt in: migrations run as a
  single Kubernetes Job, which IS the singleton process. No advisory
  lock needed at all.

Layout:
- migrations/000001_init.sql — schema-only pg_dump of the live Neon
  DB at adoption, stripped of psql-only directives that block goose's
  bookkeeping insert. Pre-goose hand-numbered migrations 002-022 had
  their effects folded into this baseline; deleted from the live tree
  but preserved in git history at 58e6997.
- Dockerfile installs `goose v3.22.1` at build time and copies the
  binary into the api image. The migrate Job reuses the api image with
  command=goose, so no separate image to build/push/version.
- deploy-k3s/manifests/migrate/job.yaml: a one-shot Job that strips
  the -pooler segment from DB_HOST (advisory lock won't survive
  pgbouncer transaction-mode), runs `goose up`, exits.
- deploy-k3s/scripts/03-deploy.sh: deletes any prior Job, applies the
  fresh one, `kubectl wait --for=condition=complete --timeout=10m`,
  then proceeds with api/worker rollout. Job failure aborts the deploy
  before any new app pod sees a stale schema.
- internal/database/database.go::RequireSchemaApplied checks
  goose_db_version on startup. api/worker refuse to boot if the
  table is missing or its latest row has is_applied=false — the
  fail-fast for "operator forgot to run migrate."
- Makefile: migrate-up / migrate-down / migrate-status / migrate-new
  for local workflow.

Production DB was bootstrapped manually:
  $ goose -dir migrations postgres "$DSN" version  # creates table
  $ psql ... -c "INSERT INTO goose_db_version (version_id, is_applied, tstamp) VALUES (1, true, NOW());"

Smoke test against fresh Postgres locally: 50 user tables created in
284ms via `goose up`, version_id=1 + is_applied=t recorded.

Verified the local goose CLI talks to prod successfully:
  $ goose ... status
  Applied At                  Migration
  =======================================
  Mon Apr 27 03:43:55 2026 -- 000001_init.sql

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Dockerfile

@@ -49,6 +49,12 @@ RUN CGO_ENABLED=0 GOOS=linux GOARCH=${TARGETARCH} go build -ldflags="-w -s" -o /
 # Build the worker binary
 RUN CGO_ENABLED=0 GOOS=linux GOARCH=${TARGETARCH} go build -ldflags="-w -s" -o /app/worker ./cmd/worker
 
+# Install goose CLI for production migrations. Pinned to a specific version
+# so an upstream behavioural change can't break a deploy unannounced.
+# Bumping is a deliberate, reviewable diff.
+RUN CGO_ENABLED=0 GOOS=linux GOARCH=${TARGETARCH} \
+    go install github.com/pressly/goose/v3/cmd/goose@v3.22.1
+
 # Base runtime stage for Go services
 FROM alpine:3.19 AS go-base
 
@@ -64,6 +70,9 @@ WORKDIR /app
 # Copy all binaries from builder
 COPY --from=builder /app/api /app/api
 COPY --from=builder /app/worker /app/worker
+# goose is the migration runner — same image is reused as the migrate Job
+# entrypoint via `command: ["/usr/local/bin/goose", ...]`.
+COPY --from=builder /go/bin/goose /usr/local/bin/goose
 
 # Copy templates directory
 COPY --from=builder /app/templates /app/templates