feat(notifications): requests and flags reach the people who act on them (#5340)

- A new request still pending after any auto-approval notifies the
  admins (request_pending), but not the requester if they are an admin.
  A request that dedups into one already in flight is not announced again.
- Approving or rejecting a request notifies the requester, and a
  rejection carries the admin's notes as the reason. An admin deciding
  their own request gets nothing.
- The reconciler notifies the requester when their request arrives
  (request_completed), linking the matched album or artist.
- A request the re-acquisition sweeper files and cannot approve itself
  notifies the admins.
- A quarantine flag notifies every admin except the flagger, naming the
  track, the flagger and the reason.

lidarrrequests.Service.CreateTracked reports whether a request was
inserted or deduped; Create wraps it.

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

internal/api/api.go

@@ -23,6 +23,7 @@ import (
 	"git.fabledsword.com/bvandeusen/minstrel/internal/lidarrrequests"
 	"git.fabledsword.com/bvandeusen/minstrel/internal/mailer"
 	"git.fabledsword.com/bvandeusen/minstrel/internal/netsettings"
+	"git.fabledsword.com/bvandeusen/minstrel/internal/notifications"
 	"git.fabledsword.com/bvandeusen/minstrel/internal/playevents"
 	"git.fabledsword.com/bvandeusen/minstrel/internal/playlists"
 	"git.fabledsword.com/bvandeusen/minstrel/internal/reacquisition"
@@ -61,6 +62,7 @@ func Mount(r chi.Router, pool *pgxpool.Pool, logger *slog.Logger, events *playev
 		dataDir:             dataDir,
 		mailer:              sender,
 		eventbus:            bus,
+		notifier:            notifications.New(pool, bus, logger.With("component", "notifications")),
 		playlistScheduler:   playlistScheduler,
 		streamSecret:        streamSecret,
 		netSettings:         netSettings,
@@ -333,20 +335,23 @@ type handlers struct {
 	// librarySize memoises the track count that sizes the candidate pool
 	// (#3880). Held here rather than counted per request: the count is a
 	// full table scan, and library size only moves when a scan runs.
-	librarySize       *recommendation.LibrarySize
-	lidarrCfg         *lidarrconfig.Service
-	lidarrRequests    *lidarrrequests.Service
-	lidarrQuarantine  *lidarrquarantine.Service
-	tracks            *tracks.Service
-	playlists         *playlists.Service
-	coverart          *coverart.Enricher
-	coverSettings     *coverart.SettingsService
-	tagSettings       *tags.SettingsService
-	scanner           *library.Scanner
-	scanCfg           library.RunScanConfig
-	dataDir           string
-	mailer            mailer.Sender
-	eventbus          *eventbus.Bus
+	librarySize      *recommendation.LibrarySize
+	lidarrCfg        *lidarrconfig.Service
+	lidarrRequests   *lidarrrequests.Service
+	lidarrQuarantine *lidarrquarantine.Service
+	tracks           *tracks.Service
+	playlists        *playlists.Service
+	coverart         *coverart.Enricher
+	coverSettings    *coverart.SettingsService
+	tagSettings      *tags.SettingsService
+	scanner          *library.Scanner
+	scanCfg          library.RunScanConfig
+	dataDir          string
+	mailer           mailer.Sender
+	eventbus         *eventbus.Bus
+	// notifier writes the notifications inbox (M489). Nil-safe: a nil
+	// notifier records nothing, which is what most handler tests want.
+	notifier          *notifications.Notifier
 	playlistScheduler *playlists.Scheduler
 	// reacqSettings is the DB-backed policy for auto re-acquisition of
 	// missing files (milestone #290) — grace window, backoff, attempt caps.