feat(search): add PostgreSQL FTS on AI sessions with Command Palette integration

- Migration: add generated tsvector column + GIN index on ai_sessions (problem_summary, resolution_summary, escalation_reason, problem_domain)
- Backend: wire FTS into list_sessions q param; add GET /ai-sessions/search endpoint returning AISessionSearchResult (registered before /{session_id} to avoid UUID routing conflict)
- Frontend: add AISessionSearchResult type, aiSessionsApi.search() method, and Command Palette group "FlowPilot Sessions" using Zap icon navigating to /pilot/:id

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/alembic/versions/dbf67047d4c8_add_full_text_search_vector_to_ai_.py

@@ -0,0 +1,43 @@
+"""add full-text search vector to ai_sessions
+
+Revision ID: dbf67047d4c8
+Revises: 49150866ae44
+Create Date: 2026-03-20 03:36:29.910843
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = 'dbf67047d4c8'
+down_revision: Union[str, None] = '49150866ae44'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    # Add generated tsvector column for full-text search
+    # Indexes: problem_summary, resolution_summary, escalation_reason, problem_domain
+    op.execute("""
+        ALTER TABLE ai_sessions ADD COLUMN IF NOT EXISTS search_vector tsvector
+        GENERATED ALWAYS AS (
+            to_tsvector('english',
+                coalesce(problem_summary, '') || ' ' ||
+                coalesce(resolution_summary, '') || ' ' ||
+                coalesce(escalation_reason, '') || ' ' ||
+                coalesce(problem_domain, ''))
+        ) STORED
+    """)
+    # Add GIN index for fast FTS lookups
+    op.execute("""
+        CREATE INDEX IF NOT EXISTS idx_ai_sessions_search
+        ON ai_sessions USING gin(search_vector)
+    """)
+
+
+def downgrade() -> None:
+    op.execute("DROP INDEX IF EXISTS idx_ai_sessions_search")
+    op.execute("ALTER TABLE ai_sessions DROP COLUMN IF EXISTS search_vector")