fix: Add platform_post_id column per Gemini review

- Added platform_post_id VARCHAR(255) for Phase 2 API matching
- Added composite index on (platform, platform_post_id)
- Future-proofs manual entries for automatic API sync

Chronicler #76 (per Gemini consultation)

services/arbiter-3.0/migrations/108_social_analytics.sql

@@ -37,7 +37,10 @@ CREATE TABLE social_posts (
     -- Metadata
     created_at TIMESTAMP DEFAULT NOW(),
     updated_at TIMESTAMP DEFAULT NOW(),
-    notes TEXT
+    notes TEXT,
+    
+    -- Platform native ID (for Phase 2 API matching)
+    platform_post_id VARCHAR(255)
 );
 
 -- Account-level snapshots (weekly or on-demand)
@@ -65,6 +68,7 @@ CREATE TABLE social_account_snapshots (
 -- Indexes for common queries
 CREATE INDEX idx_social_posts_platform ON social_posts(platform);
 CREATE INDEX idx_social_posts_posted_at ON social_posts(posted_at DESC);
+CREATE INDEX idx_social_posts_platform_post_id ON social_posts(platform, platform_post_id);
 CREATE INDEX idx_social_snapshots_platform_date ON social_account_snapshots(platform, snapshot_date DESC);
 
 -- Comments