feat(db): Add SQLite database migrations for GitLab data model

Implements a comprehensive relational schema for storing GitLab data
with full audit trail and raw payload preservation.

Migration 001_initial.sql establishes core metadata tables:
- projects: Tracked GitLab projects with paths and namespace
- sync_watermarks: Cursor-based incremental sync state per project
- schema_migrations: Migration tracking with checksums for integrity

Migration 002_issues.sql creates the issues data model:
- issues: Core issue data with timestamps, author, state, counts
- labels: Project-specific label definitions with colors/descriptions
- issue_labels: Many-to-many junction for issue-label relationships
- milestones: Project milestones with state and due dates
- discussions: Threaded discussions linked to issues/MRs
- notes: Individual notes within discussions with full metadata
- raw_payloads: Compressed original API responses keyed by entity

Migration 003_indexes.sql adds performance indexes:
- Covering indexes for common query patterns (state, updated_at)
- Composite indexes for filtered queries (project + state)

Migration 004_discussions_payload.sql extends discussions:
- Adds raw_payload column for discussion-level API preservation
- Enables debugging and data recovery from original responses

Migration 005_assignees_milestone_duedate.sql completes the model:
- issue_assignees: Many-to-many for multiple assignees per issue
- Adds milestone_id, due_date columns to issues table
- Indexes for assignee and milestone filtering

Schema supports both incremental sync and full historical queries.

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

migrations/005_assignees_milestone_duedate.sql

@@ -0,0 +1,43 @@
+-- Migration 005: Add assignees, milestone, and due_date support
+-- Schema version: 5
+
+-- Add new columns to issues table
+ALTER TABLE issues ADD COLUMN due_date TEXT;           -- YYYY-MM-DD format, nullable
+ALTER TABLE issues ADD COLUMN milestone_id INTEGER;    -- Local milestone ID (FK to milestones.id)
+ALTER TABLE issues ADD COLUMN milestone_title TEXT;    -- Denormalized for quick display
+
+-- Milestones table (captures key fields for filtering and display)
+CREATE TABLE IF NOT EXISTS milestones (
+    id INTEGER PRIMARY KEY,
+    gitlab_id INTEGER NOT NULL,
+    project_id INTEGER NOT NULL REFERENCES projects(id) ON DELETE CASCADE,
+    iid INTEGER NOT NULL,                              -- Project-scoped milestone number
+    title TEXT NOT NULL,
+    description TEXT,
+    state TEXT,                                        -- 'active' or 'closed'
+    due_date TEXT,                                     -- YYYY-MM-DD
+    web_url TEXT,
+    UNIQUE(project_id, gitlab_id)
+);
+
+CREATE INDEX IF NOT EXISTS idx_milestones_project ON milestones(project_id);
+CREATE INDEX IF NOT EXISTS idx_milestones_state ON milestones(project_id, state);
+
+-- Issue assignees junction table (issues can have multiple assignees)
+CREATE TABLE IF NOT EXISTS issue_assignees (
+    issue_id INTEGER NOT NULL REFERENCES issues(id) ON DELETE CASCADE,
+    username TEXT NOT NULL,
+    PRIMARY KEY (issue_id, username)
+);
+
+CREATE INDEX IF NOT EXISTS idx_issue_assignees_username ON issue_assignees(username);
+
+-- Index for due_date filtering
+CREATE INDEX IF NOT EXISTS idx_issues_due_date ON issues(due_date);
+
+-- Index for milestone filtering
+CREATE INDEX IF NOT EXISTS idx_issues_milestone ON issues(milestone_id);
+
+-- Update schema version
+INSERT INTO schema_version (version, applied_at, description)
+VALUES (5, strftime('%s', 'now') * 1000, 'Add assignees, milestone, and due_date support');