feat: M3 — automatic channel tagging (language + topic system tags)

- Tag and ChannelTag models + migration 0003 (partial unique indexes split
  system vs per-user tag names)
- Offline language detection (py3langid) constrained to a curated language set,
  with the channel's declared default language as a strong prior
- Topic tags mapped from YouTube topicDetails + dominant video category; the
  generic "Lifestyle" catch-all is intentionally dropped
- System (auto) tags are regenerated idempotently and never touch user tags;
  orphaned system tags are cleaned up
- GET /api/tags and admin POST /api/tags/recompute; scheduled autotag job

backend/app/models.py

@@ -5,6 +5,7 @@ from sqlalchemy import (
     Boolean,
     Date,
     DateTime,
+    Float,
     ForeignKey,
     Integer,
     JSON,
@@ -155,6 +156,51 @@ class Video(Base):
     channel: Mapped["Channel"] = relationship(back_populates="videos")
 
 
+class Tag(Base):
+    """A label. System tags (user_id NULL) are computed automatically and shared by all
+    users; user tags (user_id set) are personal. category groups them in the UI."""
+
+    __tablename__ = "tags"
+
+    id: Mapped[int] = mapped_column(primary_key=True)
+    user_id: Mapped[int | None] = mapped_column(
+        ForeignKey("users.id", ondelete="CASCADE"), index=True
+    )
+    name: Mapped[str] = mapped_column(String(64))
+    color: Mapped[str | None] = mapped_column(String(16))
+    # language | topic | other
+    category: Mapped[str] = mapped_column(
+        String(16), default="other", server_default="other"
+    )
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), server_default=func.now()
+    )
+
+
+class ChannelTag(Base):
+    """Links a tag to a channel. source='auto' rows are managed by the auto-tagger and
+    regenerated freely; source='user' rows are never touched by the system."""
+
+    __tablename__ = "channel_tags"
+    __table_args__ = (
+        UniqueConstraint("channel_id", "tag_id", "user_id", name="uq_channel_tag"),
+    )
+
+    id: Mapped[int] = mapped_column(primary_key=True)
+    channel_id: Mapped[str] = mapped_column(
+        ForeignKey("channels.id", ondelete="CASCADE"), index=True
+    )
+    tag_id: Mapped[int] = mapped_column(
+        ForeignKey("tags.id", ondelete="CASCADE"), index=True
+    )
+    # NULL = system/auto link visible to everyone; set = a specific user's link.
+    user_id: Mapped[int | None] = mapped_column(
+        ForeignKey("users.id", ondelete="CASCADE")
+    )
+    source: Mapped[str] = mapped_column(String(8), default="auto", server_default="auto")
+    confidence: Mapped[float | None] = mapped_column(Float)
+
+
 class ApiQuotaUsage(Base):
     """Tracks YouTube Data API units spent per Pacific-time day (the quota resets at
     midnight Pacific). The whole app shares one daily budget."""