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/007_documents.sql

@@ -0,0 +1,84 @@
+-- Migration 007: Documents, Document Labels, Document Paths, Dirty Sources, Pending Discussion Fetches
+-- Schema version: 7
+-- Adds CP3 document storage and queue tables for search pipeline
+
+-- Unified searchable documents (derived from issues/MRs/discussions)
+CREATE TABLE documents (
+  id INTEGER PRIMARY KEY,
+  source_type TEXT NOT NULL CHECK (source_type IN ('issue','merge_request','discussion')),
+  source_id INTEGER NOT NULL,    -- local DB id in the source table
+  project_id INTEGER NOT NULL REFERENCES projects(id),
+  author_username TEXT,          -- for discussions: first note author
+  label_names TEXT,              -- JSON array (display/debug only)
+  created_at INTEGER,            -- ms epoch UTC
+  updated_at INTEGER,            -- ms epoch UTC
+  url TEXT,
+  title TEXT,                    -- null for discussions
+  content_text TEXT NOT NULL,    -- canonical text for embedding/search
+  content_hash TEXT NOT NULL,    -- SHA-256 for change detection
+  labels_hash TEXT NOT NULL DEFAULT '',  -- SHA-256 over sorted labels (write optimization)
+  paths_hash TEXT NOT NULL DEFAULT '',   -- SHA-256 over sorted paths (write optimization)
+  is_truncated INTEGER NOT NULL DEFAULT 0,
+  truncated_reason TEXT CHECK (
+    truncated_reason IN (
+      'token_limit_middle_drop','single_note_oversized','first_last_oversized',
+      'hard_cap_oversized'
+    )
+    OR truncated_reason IS NULL
+  ),
+  UNIQUE(source_type, source_id)
+);
+
+CREATE INDEX idx_documents_project_updated ON documents(project_id, updated_at);
+CREATE INDEX idx_documents_author ON documents(author_username);
+CREATE INDEX idx_documents_source ON documents(source_type, source_id);
+CREATE INDEX idx_documents_hash ON documents(content_hash);
+
+-- Fast label filtering (indexed exact-match)
+CREATE TABLE document_labels (
+  document_id INTEGER NOT NULL REFERENCES documents(id) ON DELETE CASCADE,
+  label_name TEXT NOT NULL,
+  PRIMARY KEY(document_id, label_name)
+) WITHOUT ROWID;
+CREATE INDEX idx_document_labels_label ON document_labels(label_name);
+
+-- Fast path filtering (DiffNote file paths)
+CREATE TABLE document_paths (
+  document_id INTEGER NOT NULL REFERENCES documents(id) ON DELETE CASCADE,
+  path TEXT NOT NULL,
+  PRIMARY KEY(document_id, path)
+) WITHOUT ROWID;
+CREATE INDEX idx_document_paths_path ON document_paths(path);
+
+-- Queue for incremental document regeneration (with retry tracking)
+-- Uses next_attempt_at for index-friendly backoff queries
+CREATE TABLE dirty_sources (
+  source_type TEXT NOT NULL CHECK (source_type IN ('issue','merge_request','discussion')),
+  source_id INTEGER NOT NULL,
+  queued_at INTEGER NOT NULL,    -- ms epoch UTC
+  attempt_count INTEGER NOT NULL DEFAULT 0,
+  last_attempt_at INTEGER,
+  last_error TEXT,
+  next_attempt_at INTEGER,       -- ms epoch UTC; NULL means ready immediately
+  PRIMARY KEY(source_type, source_id)
+);
+CREATE INDEX idx_dirty_sources_next_attempt ON dirty_sources(next_attempt_at);
+
+-- Resumable queue for dependent discussion fetching
+-- Uses next_attempt_at for index-friendly backoff queries
+CREATE TABLE pending_discussion_fetches (
+  project_id INTEGER NOT NULL REFERENCES projects(id),
+  noteable_type TEXT NOT NULL,            -- 'Issue' | 'MergeRequest'
+  noteable_iid INTEGER NOT NULL,
+  queued_at INTEGER NOT NULL,             -- ms epoch UTC
+  attempt_count INTEGER NOT NULL DEFAULT 0,
+  last_attempt_at INTEGER,
+  last_error TEXT,
+  next_attempt_at INTEGER,                -- ms epoch UTC; NULL means ready immediately
+  PRIMARY KEY(project_id, noteable_type, noteable_iid)
+);
+CREATE INDEX idx_pending_discussions_next_attempt ON pending_discussion_fetches(next_attempt_at);
+
+-- Update schema version
+INSERT INTO schema_version (version, applied_at, description)
+VALUES (7, strftime('%s', 'now') * 1000, 'Documents, labels, paths, dirty sources, pending discussion fetches');