Fix source-missing auto-merge and remove Pink Floyd examples from prompts

Auto-merge: when ingest pipeline detects "source file missing", now checks
if the track already exists in the library by file_hash. If so, marks the
pending entry as 'merged' instead of 'error' — avoiding stale error entries
for files that were already successfully ingested in a previous run.

Prompts: replaced Pink Floyd/The Wall/Have a Cigar examples in both
normalize.txt and merge.txt with Deep Purple examples. The LLM was using
these famous artist/album/track names as fallback output when raw metadata
was empty or ambiguous, causing hallucinated metadata like
"artist: Pink Floyd, title: Have a Cigar" for completely unrelated tracks.

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

furumi-agent/prompts/merge.txt

@@ -4,14 +4,14 @@ You are a music library artist merge assistant. You will receive a list of artis
 
 You will receive a structured list like:
 
-### Artist ID 42: "pink floyd"
-  Album ID 10: "the wall" (1979)
-    - 01. "In the Flesh?" [track_id=100]
-    - 02. "The Thin Ice" [track_id=101]
+### Artist ID 42: "deep purple"
+  Album ID 10: "machine head" (1972)
+    - 01. "Highway Star" [track_id=100]
+    - 02. "Maybe I'm a Leo" [track_id=101]
 
-### Artist ID 43: "Pink Floyd"
-  Album ID 11: "Wish You Were Here" (1975)
-    - 01. "Shine On You Crazy Diamond (Parts I-V)" [track_id=200]
+### Artist ID 43: "Deep Purple"
+  Album ID 11: "Burn" (1974)
+    - 01. "Burn" [track_id=200]
 
 ## Your task
 
@@ -20,7 +20,7 @@ Determine if the artists are duplicates and produce a merge plan.
 ## Rules
 
 ### 1. Canonical artist name
-- Use correct capitalization and canonical spelling (e.g., "pink floyd" → "Pink Floyd", "AC DC" → "AC/DC").
+- Use correct capitalization and canonical spelling (e.g., "deep purple" → "Deep Purple", "AC DC" → "AC/DC").
 - If the database already contains an artist with a well-formed name, prefer that exact form.
 - If one artist has clearly more tracks or albums, their name spelling may be more authoritative.
 - Fix obvious typos or casing errors.
@@ -54,7 +54,7 @@ Determine if the artists are duplicates and produce a merge plan.
 
 You MUST respond with a single JSON object, no markdown fences, no extra text:
 
-{"canonical_artist_name": "...", "winner_artist_id": 42, "album_mappings": [{"source_album_id": 10, "canonical_name": "The Wall", "merge_into_album_id": null}, {"source_album_id": 11, "canonical_name": "Wish You Were Here", "merge_into_album_id": null}], "notes": "..."}
+{"canonical_artist_name": "...", "winner_artist_id": 42, "album_mappings": [{"source_album_id": 10, "canonical_name": "Machine Head", "merge_into_album_id": null}, {"source_album_id": 11, "canonical_name": "Burn", "merge_into_album_id": null}], "notes": "..."}
 
 - `canonical_artist_name`: the single correct name for this artist after merging.
 - `winner_artist_id`: the integer ID of the artist whose record survives (must be one of the IDs provided).