docs: add missing content from summary.md (API reference, Android app, file structure, LLM internals)

- docs/api-reference.md: complete REST API endpoint reference (~60+ routes)
- docs/android-app.md: Flutter companion app stack, architecture, feature status
- docs/architecture.md: detailed file-by-file reference for all backend services and frontend components
- docs/features.md: LLM pipeline internals (intent routing, tool loop, duplicate guards, image search, research pipeline), roadmap
- docs/development.md: full migration chain (0001–0026) with naming and caveats
- README.md: link to new api-reference and android-app docs

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/development.md

@@ -97,6 +97,41 @@ Required secrets (repo → Settings → Secrets → Actions):
 - `REGISTRY_USER` — Forgejo username
 - `REGISTRY_TOKEN` — Forgejo PAT with `write:packages` scope
 
+## Migration Chain
+
+Current migration sequence (all idempotent raw SQL):
+
+```
+0001 create_notes_table
+0002 create_tasks_table
+0003 task_note_companion (data migration)
+0004 merge_tasks_into_notes
+0005 add_chat_tables
+0006 add_settings_table
+0007 add_title_and_updated_at_indexes
+0008 add_users_and_user_id
+0009 add_message_status
+0010 add_app_logs_table
+0011 add_password_reset_tokens
+0012 add_invitation_tokens
+0013 add_tool_calls_to_messages
+0014 add_note_embeddings
+0015 add_oauth_fields
+0016 add_image_cache
+0017 add_projects
+0018 add_push_subscriptions
+0019 add_note_parent_and_project_fields
+0020 add_milestones
+0021 add_task_logs
+0022 add_note_versions_and_drafts
+0023 add_tags_to_note_versions
+0024 add_session_version
+0025 add_sharing_and_notifications
+0026 add_briefing_tables
+```
+
+**Important:** Do NOT use `op.create_table()` or `sa.Enum()` — SQLAlchemy's event system can fire `CREATE TYPE` even with `create_type=False`, causing failures on re-run. Always use raw SQL with `IF NOT EXISTS` / `DO $$ BEGIN ... EXCEPTION WHEN duplicate_object` guards.
+
 ## Project Conventions
 
 ### Backend