fix: add FTS5 search index to prevent 69% search failure rate (v2.18.5)

Fixes production search failures where 69% of user searches returned zero
results for critical nodes (webhook, merge, split batch) despite nodes
existing in database.

Root Cause:
- schema.sql missing nodes_fts FTS5 virtual table
- No validation to detect empty database or missing FTS5
- rebuild.ts used schema without search index
- Result: 9 of 13 searches failed in production

Changes:
1. Schema Updates (src/database/schema.sql):
   - Added nodes_fts FTS5 virtual table with full-text indexing
   - Added INSERT/UPDATE/DELETE triggers for auto-sync
   - Indexes: node_type, display_name, description, documentation, operations

2. Database Validation (src/scripts/rebuild.ts):
   - Added empty database detection (fails if zero nodes)
   - Added FTS5 existence and synchronization validation
   - Added searchability tests for critical nodes
   - Added minimum node count check (500+)

3. Runtime Health Checks (src/mcp/server.ts):
   - Database health validation on first access
   - Detects empty database with clear error
   - Detects missing FTS5 with actionable warning

4. Test Suite (53 new tests):
   - tests/integration/database/node-fts5-search.test.ts (14 tests)
   - tests/integration/database/empty-database.test.ts (14 tests)
   - tests/integration/ci/database-population.test.ts (25 tests)

5. Database Rebuild:
   - data/nodes.db rebuilt with FTS5 index
   - 535 nodes fully synchronized with FTS5

Impact:
- ✅ All critical searches now work (webhook, merge, split, code, http)
- ✅ FTS5 provides fast ranked search (< 100ms)
- ✅ Clear error messages if database empty
- ✅ CI validates committed database integrity
- ✅ Runtime health checks detect issues immediately

Performance:
- FTS5 search: < 100ms for typical queries
- LIKE fallback: < 500ms (unchanged, still functional)

Testing: LIKE search investigation revealed it was perfectly functional,
only failed because database was empty. No changes needed.

Related: Issue #296 Part 2 (Part 1: v2.18.4 fixed adapter bypass)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

src/database/schema.sql

@@ -25,6 +25,40 @@ CREATE INDEX IF NOT EXISTS idx_package ON nodes(package_name);
 CREATE INDEX IF NOT EXISTS idx_ai_tool ON nodes(is_ai_tool);
 CREATE INDEX IF NOT EXISTS idx_category ON nodes(category);
 
+-- FTS5 full-text search index for nodes
+CREATE VIRTUAL TABLE IF NOT EXISTS nodes_fts USING fts5(
+  node_type,
+  display_name,
+  description,
+  documentation,
+  operations,
+  content=nodes,
+  content_rowid=rowid
+);
+
+-- Triggers to keep FTS5 in sync with nodes table
+CREATE TRIGGER IF NOT EXISTS nodes_fts_insert AFTER INSERT ON nodes
+BEGIN
+  INSERT INTO nodes_fts(rowid, node_type, display_name, description, documentation, operations)
+  VALUES (new.rowid, new.node_type, new.display_name, new.description, new.documentation, new.operations);
+END;
+
+CREATE TRIGGER IF NOT EXISTS nodes_fts_update AFTER UPDATE ON nodes
+BEGIN
+  UPDATE nodes_fts
+  SET node_type = new.node_type,
+      display_name = new.display_name,
+      description = new.description,
+      documentation = new.documentation,
+      operations = new.operations
+  WHERE rowid = new.rowid;
+END;
+
+CREATE TRIGGER IF NOT EXISTS nodes_fts_delete AFTER DELETE ON nodes
+BEGIN
+  DELETE FROM nodes_fts WHERE rowid = old.rowid;
+END;
+
 -- Templates table for n8n workflow templates
 CREATE TABLE IF NOT EXISTS templates (
   id INTEGER PRIMARY KEY,
@@ -108,5 +142,6 @@ FROM template_node_configs
 WHERE rank <= 5  -- Top 5 per node type
 ORDER BY node_type, rank;
 
--- Note: FTS5 tables are created conditionally at runtime if FTS5 is supported
--- See template-repository.ts initializeFTS5() method
+-- Note: Template FTS5 tables are created conditionally at runtime if FTS5 is supported
+-- See template-repository.ts initializeFTS5() method
+-- Node FTS5 table (nodes_fts) is created above during schema initialization