fix: address platform source review feedback

Tighten platform source persistence so legacy callers cannot silently relabel existing sessions, repair migration 24 when schema_versions drifts from the real schema, and polish the follow-up UI/error-handler review nits.

- only backfill platform_source when it is blank and raise on explicit source conflicts for an existing session
- make migration 24 verify both the sdk_sessions column and its index before treating it as applied
- expose platform_source from the functional session getters and add regression tests for source preservation and schema drift recovery
- add the required APPROVED OVERRIDE annotation for centralized HTTP error translation
- keep mobile source pills on a single horizontal row

src/services/sqlite/migrations/runner.ts

@@ -871,11 +871,13 @@ export class MigrationRunner {
    * Add platform_source column to sdk_sessions for Claude/Codex isolation (migration 24)
    */
   private addSessionPlatformSourceColumn(): void {
-    const applied = this.db.prepare('SELECT version FROM schema_versions WHERE version = ?').get(24) as SchemaVersion | undefined;
-    if (applied) return;
-
     const tableInfo = this.db.query('PRAGMA table_info(sdk_sessions)').all() as TableColumnInfo[];
     const hasColumn = tableInfo.some(col => col.name === 'platform_source');
+    const indexInfo = this.db.query('PRAGMA index_list(sdk_sessions)').all() as IndexInfo[];
+    const hasIndex = indexInfo.some(index => index.name === 'idx_sdk_sessions_platform_source');
+    const applied = this.db.prepare('SELECT version FROM schema_versions WHERE version = ?').get(24) as SchemaVersion | undefined;
+
+    if (applied && hasColumn && hasIndex) return;
 
     if (!hasColumn) {
       this.db.run(`ALTER TABLE sdk_sessions ADD COLUMN platform_source TEXT NOT NULL DEFAULT '${DEFAULT_PLATFORM_SOURCE}'`);
@@ -887,7 +889,10 @@ export class MigrationRunner {
       SET platform_source = '${DEFAULT_PLATFORM_SOURCE}'
       WHERE platform_source IS NULL OR platform_source = ''
     `);
-    this.db.run('CREATE INDEX IF NOT EXISTS idx_sdk_sessions_platform_source ON sdk_sessions(platform_source)');
+
+    if (!hasIndex) {
+      this.db.run('CREATE INDEX IF NOT EXISTS idx_sdk_sessions_platform_source ON sdk_sessions(platform_source)');
+    }
 
     this.db.prepare('INSERT OR IGNORE INTO schema_versions (version, applied_at) VALUES (?, ?)').run(24, new Date().toISOString());
   }