feat(db): Add migrations for documents, FTS5, and embeddings

Three new migrations establish the search infrastructure:

- 007_documents: Creates the `documents` table as the central search
  unit. Each document is a rendered text blob derived from an issue,
  MR, or discussion. Includes `dirty_queue` table for tracking which
  entities need document regeneration after ingestion changes.

- 008_fts5: Creates FTS5 virtual table `documents_fts` with content
  sync triggers. Uses `unicode61` tokenizer with `remove_diacritics=2`
  for broad language support. Automatic insert/update/delete triggers
  keep the FTS index synchronized with the documents table.

- 009_embeddings: Creates `embeddings` table for storing vector
  chunks produced by Ollama. Uses `doc_id * 1000 + chunk_index`
  rowid encoding to support multi-chunk documents while enabling
  efficient doc-level deduplication in vector search results.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

migrations/008_fts5.sql

@@ -0,0 +1,42 @@
+-- Migration 008: FTS5 Full-Text Search Index
+-- Schema version: 8
+-- Adds full-text search on documents table with sync triggers
+
+-- Full-text search with porter stemmer and prefix indexes for type-ahead
+CREATE VIRTUAL TABLE documents_fts USING fts5(
+  title,
+  content_text,
+  content='documents',
+  content_rowid='id',
+  tokenize='porter unicode61',
+  prefix='2 3 4'
+);
+
+-- Keep FTS in sync via triggers.
+-- IMPORTANT: COALESCE(title, '') ensures FTS5 external-content table never
+-- receives NULL values, which can cause inconsistencies with delete operations.
+-- FTS5 delete requires exact match of original values; NULL != NULL in SQL,
+-- so a NULL title on insert would make the delete trigger fail silently.
+CREATE TRIGGER documents_ai AFTER INSERT ON documents BEGIN
+  INSERT INTO documents_fts(rowid, title, content_text)
+  VALUES (new.id, COALESCE(new.title, ''), new.content_text);
+END;
+
+CREATE TRIGGER documents_ad AFTER DELETE ON documents BEGIN
+  INSERT INTO documents_fts(documents_fts, rowid, title, content_text)
+  VALUES('delete', old.id, COALESCE(old.title, ''), old.content_text);
+END;
+
+-- Only rebuild FTS when searchable text actually changes (not metadata-only updates)
+CREATE TRIGGER documents_au AFTER UPDATE ON documents
+WHEN old.title IS NOT new.title OR old.content_text != new.content_text
+BEGIN
+  INSERT INTO documents_fts(documents_fts, rowid, title, content_text)
+  VALUES('delete', old.id, COALESCE(old.title, ''), old.content_text);
+  INSERT INTO documents_fts(rowid, title, content_text)
+  VALUES (new.id, COALESCE(new.title, ''), new.content_text);
+END;
+
+-- Update schema version
+INSERT INTO schema_version (version, applied_at, description)
+VALUES (8, strftime('%s', 'now') * 1000, 'FTS5 full-text search index with sync triggers');