feat: Add custom slug support via mp-slug property

Implements custom slug handling for Micropub as specified in ADR-035.

Changes:
- Created starpunk/slug_utils.py with validation/sanitization functions
- Added RESERVED_SLUGS constant (api, admin, auth, feed, etc.)
- Modified create_note() to accept optional custom_slug parameter
- Integrated mp-slug extraction in Micropub handle_create()
- Slug sanitization: lowercase, hyphens, no special chars
- Conflict resolution: sequential numbering (-2, -3, etc.)
- Hierarchical slugs (/) rejected (deferred to v1.2.0)

Features:
- Custom slugs via Micropub's mp-slug property
- Automatic sanitization of invalid characters
- Reserved slug protection
- Sequential conflict resolution (not random)
- Clear error messages for validation failures

Part of v1.1.0 (Phase 4).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

starpunk/notes.py

@@ -134,7 +134,7 @@ def _get_existing_slugs(db) -> set[str]:
 
 
 def create_note(
-    content: str, published: bool = False, created_at: Optional[datetime] = None
+    content: str, published: bool = False, created_at: Optional[datetime] = None, custom_slug: Optional[str] = None
 ) -> Note:
     """
     Create a new note
@@ -147,6 +147,7 @@ def create_note(
         content: Markdown content for the note (must not be empty)
         published: Whether the note should be published (default: False)
         created_at: Creation timestamp (default: current UTC time)
+        custom_slug: Optional custom slug (from Micropub mp-slug property)
 
     Returns:
         Note object with all metadata and content loaded
@@ -208,20 +209,27 @@ def create_note(
 
     data_dir = Path(current_app.config["DATA_PATH"])
 
-    # 3. GENERATE UNIQUE SLUG
+    # 3. GENERATE OR VALIDATE SLUG
     # Query all existing slugs from database
     db = get_db(current_app)
     existing_slugs = _get_existing_slugs(db)
 
-    # Generate base slug from content
-    base_slug = generate_slug(content, created_at)
+    if custom_slug:
+        # Use custom slug (from Micropub mp-slug property)
+        from starpunk.slug_utils import validate_and_sanitize_custom_slug
+        success, slug, error = validate_and_sanitize_custom_slug(custom_slug, existing_slugs)
+        if not success:
+            raise InvalidNoteDataError("slug", custom_slug, error)
+    else:
+        # Generate base slug from content
+        base_slug = generate_slug(content, created_at)
 
-    # Make unique if collision
-    slug = make_slug_unique(base_slug, existing_slugs)
+        # Make unique if collision
+        slug = make_slug_unique(base_slug, existing_slugs)
 
-    # Validate final slug (defensive check)
-    if not validate_slug(slug):
-        raise InvalidNoteDataError("slug", slug, f"Generated slug is invalid: {slug}")
+        # Validate final slug (defensive check)
+        if not validate_slug(slug):
+            raise InvalidNoteDataError("slug", slug, f"Generated slug is invalid: {slug}")
 
     # 4. GENERATE FILE PATH
     note_path = generate_note_path(slug, created_at, data_dir)