refactor(server): extract the artist-alias routes into routers/artist_aliases.py (R3) (#838)

* refactor(server): extract the artist-alias routes into routers/artist_aliases.py (R3)

Second router, picked by router_scan.py: the artist-aliases / Tidy-up (P4) group
ranks at 0 monkeypatch.setattr targets and 0 helpers to relocate. 5 routes
(list/set/merge/delete aliases + raw-artist picker), all meta_db-only.

Bodies verbatim; only @app.<m> -> @router.<m> and meta_db -> appstate.meta_db.
include_router mounts at the original site; full 143-route table identical to
origin/main (paths, methods, order). No test retargets: test_artist_alias drives
via TestClient(server.app) + server.meta_db, neither of which moved.

Verified: pyflakes clean on the router; no new undefined in server.py; JSONResponse
still used in server.py (not dead); pytest 2398 passed (18 in test_artist_alias);
packaging guard green; eslint 0; boot smoke drives all 5 routes end-to-end
(set ACDC->AC/DC, read back, 400 on missing fields, delete).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* docs: credit both router extractions in the size-exemptions rationale (CodeRabbit)

---------

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

server.py

@@ -71,7 +71,7 @@ from audio_effects_db import AudioEffectsMappingDB
 # Lives in lib/ because that is the one core dir every packaging path copies.
 import appstate
 # Extracted route modules. They import `appstate`, never `server` — one-way graph.
-from routers import audio_effects
+from routers import audio_effects, artist_aliases
 import sloppak as sloppak_mod
 import drums as drums_mod
 import notation as notation_mod
@@ -4847,59 +4847,9 @@ def list_tags():
 
 
 # ── Artist aliases / Tidy-up (P4) ────────────────────────────────────────────
-# Canonicalize messy artist tags at DISPLAY ("ACDC" → "AC/DC") without touching
-# the feedpak files or the scanner-derived songs.artist. All DB-only.
-
-@app.get("/api/artist-aliases")
-def list_artist_aliases():
-    """Existing raw→canonical overrides (the Tidy-up 'current merges' list)."""
-    return {"aliases": meta_db.list_artist_aliases()}
-
-
-@app.get("/api/artists/raw")
-def list_raw_artists(limit: int = 2000):
-    """Distinct RAW artist names + song counts + current canonical — the Tidy-up
-    picker (you merge raw variants into one canonical)."""
-    return {"artists": meta_db.raw_artists(limit)}
-
-
-@app.post("/api/artist-aliases")
-def set_artist_alias(data: dict):
-    """Upsert one override: {raw_name, canonical_name, mb_artist_id?}. A self-alias
-    (raw == canonical) clears the row instead (un-merge)."""
-    raw = (data.get("raw_name") or "").strip()
-    canon = (data.get("canonical_name") or "").strip()
-    if not raw or not canon:
-        return JSONResponse({"error": "raw_name and canonical_name are required"}, 400)
-    result = meta_db.set_artist_alias(raw, canon, (data.get("mb_artist_id") or None))
-    if not result.get("ok"):
-        # Would form a cycle (raw → … → raw) — refuse rather than corrupt the chain.
-        return JSONResponse(
-            {"error": "alias would create a cycle", "raw_name": raw, "canonical_name": canon},
-            409)
-    return {"ok": True, "raw_name": raw, "canonical_name": result.get("canonical_name", canon)}
-
-
-@app.post("/api/artist-aliases/merge")
-def merge_artist_aliases(data: dict):
-    """Merge several raw artist variants into one canonical:
-    {raw_names: [...], canonical_name}. The canonical's own self-alias is skipped.
-    Returns {merged: N}."""
-    canon = (data.get("canonical_name") or "").strip()
-    raws = data.get("raw_names")
-    if not canon:
-        return JSONResponse({"error": "canonical_name is required"}, 400)
-    if not isinstance(raws, list) or not raws:
-        return JSONResponse({"error": "raw_names must be a non-empty array"}, 400)
-    n = meta_db.merge_artists(raws, canon)
-    return {"merged": n, "canonical_name": canon}
-
-
-@app.delete("/api/artist-aliases/{raw_name:path}")
-def delete_artist_alias(raw_name: str):
-    """Remove one override so that raw artist stands on its own again."""
-    meta_db.remove_artist_alias(raw_name)
-    return {"ok": True}
+# Mounted here, where these routes used to be defined (FastAPI matches in
+# registration order). Implementation in lib/routers/artist_aliases.py.
+app.include_router(artist_aliases.router)
 
 
 # ── Artist pages (launch charrette PR-B) ──────────────────────────────────────