Tag albums service: album lookup API for Home Assistant and assignment web page

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

CONTEXT.md

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+# Tag albums
+
+A small service that tells Home Assistant which album to play for a scanned RFID card, and a web page to assign albums to new cards.
+
+## Language
+
+**Tag**:
+A physical RFID card, identified by the ID the reader reports (e.g. `53-13-0B-2A-34-00-01`). Nothing is ever written to it.
+_Avoid_: card (in code), NFC tag
+
+**Assignment**:
+The link from one **Tag** to one album (artist + album) in the music library.
+_Avoid_: encoding, mapping, tag map
+
+**Unassigned tag**:
+A **Tag** the service has seen scanned but that has no **Assignment** yet.
+_Avoid_: unknown tag, new tag
+
+**Scan**:
+One report that a **Tag** was placed on a reader, as relayed by Home Assistant. A **Scan** of an **Unassigned tag** is how the service learns about it.
+
+## Relationships
+
+- A **Tag** has zero or one **Assignment**
+- An **Assignment** points to exactly one album
+- Every **Scan** is of exactly one **Tag**; the first **Scan** of an unseen **Tag** creates it as an **Unassigned tag**
+
+## Example dialogue
+
+> **Dev:** "Someone put a new card on the reader — do we need to encode it?"
+> **Domain expert:** "It's an **Unassigned tag** now; open the page and give it an **Assignment**. Nothing gets written to the card."
+
+## Flagged ambiguities
+
+- "Encode the tags" meant assigning an album to a **Tag**, not writing data onto it. Resolved: **Assignment**.