feat(tags): folksonomy tag enricher — pluggable provider chain (#1490 Step 2)

Milestone #160 Option 1, Step 2. New internal/tags package that enriches
the taste profile's tag facet beyond raw ID3 genre, built so adding a
source later is "implement TrackTagProvider + Register()" — no enricher,
settings, or schema change (operator directive).

Mirrors the internal/coverart pattern:
- Provider interface + package registry (Register/AllProviders/ByID);
  TrackTagProvider fetch capability + TestableProvider for the admin test.
- DB-backed SettingsService over new tag_provider_settings +
  tag_sources_meta (migration 0043) — enable/key/version, boot
  reconciliation, and a provider-hash bump that re-opens 'none' rows when
  the compiled-in provider set changes.
- Slim self-contained httpClient (rate-limit + retry + User-Agent), kept
  local so tag enrichment never depends on coverart internals.

Providers:
- MusicBrainz: keyless, default-ON, recording tags by MBID (rule #26 baseline).
- Last.fm: keyed, default-OFF, track.getTopTags by artist+track — opt-in
  once a key is supplied.

Enricher uses MERGE semantics (differs from coverart's first-success-wins
for a single image): unions tags across every enabled provider, caps to
top-K by weight, and stamps tag_source musicbrainz|lastfm|mixed|none.
Writes are transactional (atomic replace of track_tags).

Unit-tested without a DB: registry mechanics, provider JSON parsing +
weight normalization via httptest, and the pure merge/top-K/source-label
helpers. Wiring (startup + on-scan), integration tests, and the taste
union (Step 3) + Settings UI (Step 4) come next.

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

internal/db/migrations/0043_tag_provider_settings.down.sql

@@ -0,0 +1,3 @@
+-- Reverse 0043_tag_provider_settings.up.sql.
+DROP TABLE IF EXISTS tag_sources_meta;
+DROP TABLE IF EXISTS tag_provider_settings;

internal/db/migrations/0043_tag_provider_settings.up.sql

@@ -0,0 +1,37 @@
+-- Tag-enrichment provider settings (milestone #160, #1490 Step 2).
+--
+-- Mirrors the cover-art provider-settings substrate (migration 0018) so
+-- adding a new tag source later is "register a provider + it auto-upserts
+-- a row here" — never a schema change. The tag enricher reads the enabled
+-- set + api keys from tag_provider_settings and stamps the current version
+-- from tag_sources_meta onto tracks.tag_sources_version (added in 0042).
+--
+-- current_version starts at 1 while tracks.tag_sources_version defaults to
+-- 0 (migration 0042), so every existing track is "stale" relative to
+-- current and gets enriched on the first pass after this migrates.
+
+CREATE TABLE tag_provider_settings (
+    provider_id   text PRIMARY KEY,
+    enabled       boolean     NOT NULL DEFAULT true,
+    api_key       text,
+    display_order int         NOT NULL DEFAULT 0,
+    created_at    timestamptz NOT NULL DEFAULT now(),
+    updated_at    timestamptz NOT NULL DEFAULT now()
+);
+
+CREATE TABLE tag_sources_meta (
+    id                              boolean PRIMARY KEY DEFAULT true,
+    current_version                 int     NOT NULL DEFAULT 1,
+    last_registered_providers_hash  text    NOT NULL DEFAULT '',
+    CONSTRAINT tag_sources_meta_singleton CHECK (id = true)
+);
+INSERT INTO tag_sources_meta (id, current_version) VALUES (true, 1);
+
+-- Seed the v1 providers. MusicBrainz is the keyless always-on default;
+-- Last.fm ships disabled and is opt-in once the operator supplies a key
+-- (Step 4 Settings UI), per the no-coercive-settings rule (#26). The
+-- SettingsService also upserts these at boot, so the seed is belt-and-
+-- suspenders / documents the intended defaults.
+INSERT INTO tag_provider_settings (provider_id, enabled, display_order) VALUES
+    ('musicbrainz', true,  0),
+    ('lastfm',      false, 1);