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/database/schema.ts

@@ -76,7 +76,6 @@ export const pgMessagesTable = pgTable(
       .default("pending"),
     ai_moderation_flags: pgText("ai_moderation_flags"),
     ai_moderation_score: pgReal("ai_moderation_score"),
-    ai_moderation_raw: pgText("ai_moderation_raw"),
     ai_analysis: pgText("ai_analysis"),
     ai_categories: pgText("ai_categories"),
     ai_severity: pgText("ai_severity", {
@@ -86,8 +85,6 @@ export const pgMessagesTable = pgTable(
     ai_recommended_action: pgText("ai_recommended_action", {
       enum: ["none", "monitor", "warn", "review", "delete", "escalate"],
     }),
-    ai_policy_version: pgText("ai_policy_version"),
-    ai_evidence: pgText("ai_evidence"),
     ai_analyzed_at: pgBigint("ai_analyzed_at", { mode: "number" }),
     ai_error: pgText("ai_error"),
   },
@@ -306,7 +303,6 @@ export const sqliteMessagesTable = sqliteTable(
       .default("pending"),
     ai_moderation_flags: sqliteText("ai_moderation_flags"),
     ai_moderation_score: sqliteReal("ai_moderation_score"),
-    ai_moderation_raw: sqliteText("ai_moderation_raw"),
     ai_analysis: sqliteText("ai_analysis"),
     ai_categories: sqliteText("ai_categories"),
     ai_severity: sqliteText("ai_severity", {
@@ -316,8 +312,6 @@ export const sqliteMessagesTable = sqliteTable(
     ai_recommended_action: sqliteText("ai_recommended_action", {
       enum: ["none", "monitor", "warn", "review", "delete", "escalate"],
     }),
-    ai_policy_version: sqliteText("ai_policy_version"),
-    ai_evidence: sqliteText("ai_evidence"),
     ai_analyzed_at: sqliteInteger("ai_analyzed_at"),
     ai_error: sqliteText("ai_error"),
   },