perf(test): make schema-dump target + opt-in fast-path docs (WS-6)

Session 2.6's per-test migrate:fresh in 4 backfill test classes
replays ~120 migrations. Laravel's schema:dump produces a single
SQL file that migrate:fresh loads atomically when present —
significantly faster on environments where the host has the
\`mysql\` CLI available for Laravel's load step.

Changes:
- New Makefile target \`schema-dump\` runs mysqldump INSIDE the
  bm_mysql Docker container (no host mysqldump dependency to
  GENERATE the dump). Outputs api/database/schema/mysql-schema.sql.
- api/database/schema/.gitignore added: mysql-schema.sql is NOT
  committed by default. Laravel's auto-load on migrate:fresh
  shells out to the host's \`mysql\` CLI; on hosts without it,
  the presence of the dump file actively breaks migrate (exit 127).
  Treat the dump as opt-in per dev environment.
- CLAUDE.md "Schema dumps (opt-in fast path)" subsection added with
  the workflow: brew install mysql-client → migrate to head →
  make schema-dump → optionally commit.

phpstan-baseline.neon: removed a stale "unused use \$actor" entry
in FormSubmissionService whose underlying closure pint cleaned up
in commit 060d6f3 (Task 1).

Wall-time on this dev machine (no host mysql CLI): NO speedup,
backfill tests still ~6s per setUp. The dump file is ready for
environments that have mysql CLI. Documented as a deviation.

JSON canonicalization (commit 060d6f3) is the load-bearing
correctness fix from this branch; the schema-dump perf path is a
nice-to-have that activates per-environment.

Refs: WS-6 session 2.6 deviation #5 cleanup

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

api/database/schema/.gitignore

@@ -0,0 +1,4 @@
+# Schema dumps are environment-specific and require mysql CLI on host
+# for Laravel's auto-load. NOT committed by default. See CLAUDE.md
+# "Schema dumps (opt-in fast path)".
+mysql-schema.sql