fix(db): numbered migrations that run once and fail loudly (FLAW-13)

schema_migrations records every applied step; the former migrate() is
step 3, the baseline that builds the v0.4.1 schema and repairs older
databases. Columns are only added when missing, so every other error now
stops the start with a clear message instead of being swallowed, and the
cleanup statements no longer run on each start. migrations/001_init.sql
was never read and is gone.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Dockerfile

@@ -36,7 +36,6 @@ RUN go get github.com/go-chi/chi/v5@v5.2.0 \
 # Copy application source & compiled frontend SPA assets for go:embed
 COPY cmd/ ./cmd/
 COPY internal/ ./internal/
-COPY migrations/ ./migrations/
 COPY --from=frontend-builder /app/frontend/dist ./cmd/server/dist
 
 # Run go mod tidy, static analysis (go vet), unit tests (go test), and compile final binary