feat(tags): system tags — is_system column, seeded hygiene tags, protection guards

Training hygiene step 1 (milestone #128). Migration 0075 adds
tag.is_system and seeds wip / banner / editor screenshot (kind=general),
ADOPTING an existing same-(name,kind) tag case-insensitively instead of
duplicating. These rows drive the upcoming training exclusions, so they
are protected: rename and merge-away refuse system tags (merge-INTO
stays allowed — folding an operator's old hygiene tag into the system
row is the intended move; merge is the only tag-delete path, so that
guard covers deletion). is_system rides every tag serialization.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CDgx8bQS5YrGRK76v8HUnM

backend/app/models/tag.py

@@ -10,6 +10,7 @@ from datetime import datetime
 from enum import StrEnum
 
 from sqlalchemy import (
+    Boolean,
     CheckConstraint,
     Column,
     DateTime,
@@ -17,6 +18,7 @@ from sqlalchemy import (
     Integer,
     String,
     Table,
+    false,
     func,
 )
 from sqlalchemy import (
@@ -74,6 +76,14 @@ class Tag(Base):
     fandom_id: Mapped[int | None] = mapped_column(
         ForeignKey("tag.id", ondelete="SET NULL"), nullable=True, index=True
     )
+    # System tags ship with FC (wip / banner / editor screenshot, seeded in
+    # migration 0075) and drive the training-hygiene exclusions: images
+    # carrying one are excluded from OTHER concepts' head training and from
+    # CCIP identity references. The mechanism keys on these exact rows, so
+    # they're protected from rename/merge-away/re-fandom in TagService.
+    is_system: Mapped[bool] = mapped_column(
+        Boolean, nullable=False, default=False, server_default=false()
+    )
 
     created_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True), nullable=False, server_default=func.now()