refactor: provider-agnostic bot command system + Gitea commands

Refactored the monolithic command handler (707 lines) into a pluggable
provider-handler architecture:

- Abstract ProviderCommandHandler interface (base.py)
- Handler dispatch registry routes commands by provider type
- Extracted all Immich logic into ImmichCommandHandler
- New GiteaCommandHandler with /status, /repos, /issues, /prs, /commits
- Multi-provider routing: groups context by provider type, finds handler
- handler.py reduced to ~280 line thin orchestrator

Gitea commands:
- Extended GiteaClient with get_repo_issues, get_repo_pulls, get_repo_commits
- 30 Jinja2 command templates (15 EN + 15 RU)
- Gitea capabilities updated with 6 commands + 15 command_slots
- Default command config + command template config seeded on startup
- Rate limiting: Gitea API commands share "api" category (15s cooldown)

Also:
- Command configs API accepts "gitea" provider type
- System command configs (user_id=0) visible to all users
- Webhook URL shown on Gitea provider card and edit form
- Scan interval hidden for webhook-based providers

packages/server/src/notify_bridge_server/api/command_configs.py

@@ -43,9 +43,12 @@ async def list_command_configs(
     user: User = Depends(get_current_user),
     session: AsyncSession = Depends(get_session),
 ):
-    """List all command configs for the current user."""
+    """List all command configs for the current user (including system defaults)."""
+    from sqlmodel import or_
     result = await session.exec(
-        select(CommandConfig).where(CommandConfig.user_id == user.id)
+        select(CommandConfig).where(
+            or_(CommandConfig.user_id == user.id, CommandConfig.user_id == 0)
+        )
     )
     return [_config_response(c) for c in result.all()]
 
@@ -58,7 +61,7 @@ async def create_command_config(
 ):
     """Create a new command config."""
     # Validate provider_type
-    valid_types = ("immich",)
+    valid_types = ("immich", "gitea")
     if body.provider_type not in valid_types:
         raise HTTPException(
             status_code=400,
@@ -159,7 +162,7 @@ async def _get_user_config(
     session: AsyncSession, config_id: int, user_id: int
 ) -> CommandConfig:
     config = await session.get(CommandConfig, config_id)
-    if not config or config.user_id != user_id:
+    if not config or (config.user_id != user_id and config.user_id != 0):
         raise HTTPException(status_code=404, detail="Command config not found")
     return config