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/alembic/versions/0003_tags.py

@@ -0,0 +1,89 @@
+"""tags and channel_tags
+
+Revision ID: 0003_tags
+Revises: 0002_ingest_core
+Create Date: 2026-06-11
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+revision: str = "0003_tags"
+down_revision: Union[str, None] = "0002_ingest_core"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        "tags",
+        sa.Column("id", sa.Integer(), primary_key=True),
+        sa.Column(
+            "user_id",
+            sa.Integer(),
+            sa.ForeignKey("users.id", ondelete="CASCADE"),
+            nullable=True,
+        ),
+        sa.Column("name", sa.String(length=64), nullable=False),
+        sa.Column("color", sa.String(length=16), nullable=True),
+        sa.Column("category", sa.String(length=16), nullable=False, server_default="other"),
+        sa.Column(
+            "created_at",
+            sa.DateTime(timezone=True),
+            server_default=sa.func.now(),
+            nullable=False,
+        ),
+    )
+    op.create_index("ix_tags_user_id", "tags", ["user_id"])
+    # System tag names unique among system tags; user tag names unique per user.
+    op.create_index(
+        "uq_tags_system_name",
+        "tags",
+        ["name"],
+        unique=True,
+        postgresql_where=sa.text("user_id IS NULL"),
+    )
+    op.create_index(
+        "uq_tags_user_name",
+        "tags",
+        ["user_id", "name"],
+        unique=True,
+        postgresql_where=sa.text("user_id IS NOT NULL"),
+    )
+
+    op.create_table(
+        "channel_tags",
+        sa.Column("id", sa.Integer(), primary_key=True),
+        sa.Column(
+            "channel_id",
+            sa.String(length=32),
+            sa.ForeignKey("channels.id", ondelete="CASCADE"),
+            nullable=False,
+        ),
+        sa.Column(
+            "tag_id",
+            sa.Integer(),
+            sa.ForeignKey("tags.id", ondelete="CASCADE"),
+            nullable=False,
+        ),
+        sa.Column(
+            "user_id",
+            sa.Integer(),
+            sa.ForeignKey("users.id", ondelete="CASCADE"),
+            nullable=True,
+        ),
+        sa.Column("source", sa.String(length=8), nullable=False, server_default="auto"),
+        sa.Column("confidence", sa.Float(), nullable=True),
+        sa.UniqueConstraint("channel_id", "tag_id", "user_id", name="uq_channel_tag"),
+    )
+    op.create_index("ix_channel_tags_channel_id", "channel_tags", ["channel_id"])
+    op.create_index("ix_channel_tags_tag_id", "channel_tags", ["tag_id"])
+
+
+def downgrade() -> None:
+    op.drop_table("channel_tags")
+    op.drop_index("uq_tags_user_name", table_name="tags")
+    op.drop_index("uq_tags_system_name", table_name="tags")
+    op.drop_index("ix_tags_user_id", table_name="tags")
+    op.drop_table("tags")