refactor: remove unused AI columns from messages table

Drop 3 columns that were written but never read:
- ai_moderation_raw: raw LLM JSON response (~KB per message, never consumed)
- ai_policy_version: hardcoded 'default-2026-05-30', never used for decisions
- ai_evidence: JSON evidence array, never read after write

Changes:
- schema.ts: remove columns from both Postgres and SQLite table definitions
- messageStore.ts: remove from AIAnalysisUpdate interface and SET clauses
- aiAnalyzer.ts: remove from individual fallback update calls
- aiAnalysisWorker.ts: remove raw write, add missing fields (categories, severity, etc.)
- types.ts: remove from MessageRecord interface
- analysisRoutes.ts: remove from reset analysis call
- New migration: src/database/migrations/001_drop_unused_ai_columns.sql
- Migration applied to live DB: 27 columns → 24 columns

Kept ai_error (useful for future debugging, currently 0 non-null)
Kept metadata (1.9MB total, used for AI sticker/embed evidence analysis)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/routes/analysisRoutes.ts

@@ -90,7 +90,6 @@ export function createAnalysisRoutes(): Router {
         status: "pending",
         flags: null,
         score: null,
-        raw: null,
         analysis: null,
         analyzedAt: null,
         error: null,