refactor: split ensureSchemaImpl DDL into per-domain db/schema modules

[atomantic]

Summary

Splits the ~1265-line inline DDL in ensureSchemaImpl() (server/lib/db.js) into per-domain modules under server/lib/db/schema/, mirroring the arcPlanner.js → arcPlanner/* split from #1152. ensureSchemaImpl becomes a thin composer:

const upgrades = buildUpgradeDdl();   // core, tribe, humanActivity, commissions
const catalogDDL = buildCatalogDdl(); // catalog, media, universes, library, pipeline, writersRoom, lora, privacy, audit + triggers

Each domain module (core, tribe, humanActivity, commissions, catalog, media, universes, library, pipeline, writersRoom, lora, privacy, audit) exports a plain statement array; db/schema/index.js re-exports them and composes the two ordered lists. db.js drops from 1632 to 414 lines.

No behavior change — byte-identical. The modules were extracted verbatim (statement text and order unchanged), so the composed schema is identical to the pre-split version. This was verified programmatically against the arrays from the parent commit: 28 upgrade + 181 catalog statements match exactly. Order is load-bearing (FK references, the record_audit_log() function preceding its triggers, and catalog_user_types' original post-media position) and is preserved by the composer.

catalog_user_types lives in catalog.js as a separate export (catalogUserTypesDdl) because in the original array it sits after the media block, not adjacent to the other catalog tables — the composer threads it back into that exact position.

The db.catalogDdlParity.test.js and catalogTypes.test.js text-scan tests now read the schema-module sources (the DDL literals moved out of db.js); their set/membership assertions are unchanged.

New db/schema/README.md (module catalog) and db/schema/schema.test.js (composer contract: array shapes, compose order, FK-target-precedes-reference, function-before-triggers, audit-trigger pairs).

Test plan

• server/lib/db/schema/schema.test.js, db.catalogDdlParity.test.js, catalogTypes.test.js, db.guards.test.js, lib/index.test.js — all pass.
• Full server suite: 1009 files passed, 20832 tests passed, 24 db-backed suites self-skipped on the non-test portos DB (guard held). Zero failures.
• Byte-identical composed DDL verified against the pre-split arrays (28 + 181 statements match).
• Reviewed with claude, codex (ran the full suite itself, confirmed statement + order match), and ollama (local coding model) — all clean; ollama's findings were pre-existing verbatim SQL or false-positive ordering concerns, dismissed.

Closes #2832