feat(db): typed tunable system settings + SettingsRegistry

Add a typed, cached registry over the system_settings table so that
values which used to be hardcoded Go literals can be changed at runtime.

Schema (database/schema/schema.sql):
- Extend system_settings with setting_type, min_value, max_value,
  requires_restart, and category columns (all ADD COLUMN IF NOT EXISTS,
  nullable for backward compat with the original three rows).
- Seed rows for every tunable: session duration, password rules,
  login lockout, auth/device rate limits, OPDS page size, tombstone TTL,
  conversion cache TTL, sync queue interval/batch, and worker pool
  size/cap. Seed values equal the previous hardcoded literals, so
  behavior is unchanged on upgrade. ON CONFLICT DO NOTHING preserves
  any admin-modified values.

Queries (queries.sql):
- Add UpsertSystemSetting (RETURNING *) so new keys without a seed row
  can still be written through the API.
- Add GetSystemSettingFull + GetAllSystemSettingsFull returning the
  full typed row.
- Refactor CleanupExpiredRefreshTokens to take the retention window as
  a parameter (make_interval(secs => $1)) instead of the INTERVAL '7
  days' literal, so it can follow a configurable session duration.

Registry (internal/database/settings_registry.go):
- SettingsRegistry holds an in-memory cache of all known settings,
  populated by Load at startup and refreshed by Reload on writes.
- Typed domain getters (SessionDuration, PasswordRules, DeviceRateLimits,
  TombstoneTTL, OpdsPageSize, ConversionCacheTTL, SyncQueueConfig,
  WorkerPoolConfig, LoginLockout, AuthRateLimit, ...) with compiled-in
  fallback defaults and min/max clamping, so a corrupt or missing row
  can never break the app.
- SettingDefaults is the single source of truth for keys, types, bounds,
  and human descriptions; All() exposes metadata + current values for
  the admin UI/API.

The registry lives in the database package (rather than its own
internal/settings package) because a quirk in this custom go1.26.5
toolchain prevented the large handlers package from importing any
newly-created package; every consumer already imports database.

Tests: settings_registry_test.go covers default validity per type,
int clamping at both bounds, garbage-value fallback, and unknown-key
lookup.

internal/database/querier.go

@@ -1,6 +1,6 @@
 // Code generated by sqlc. DO NOT EDIT.
 // versions:
-//   sqlc v1.30.0
+//   sqlc v1.31.1
 
 package database
 
@@ -26,7 +26,7 @@ type Querier interface {
 	CheckForProgressConflicts(ctx context.Context, arg CheckForProgressConflictsParams) (int64, error)
 	// Cleanup expired OPDS tokens
 	CleanupExpiredOpdsTokens(ctx context.Context) error
-	CleanupExpiredRefreshTokens(ctx context.Context) error
+	CleanupExpiredRefreshTokens(ctx context.Context, dollar_1 float64) error
 	ClearDeviceSyncQueue(ctx context.Context, deviceID pgtype.UUID) error
 	ClearKoboShelf(ctx context.Context, deviceID pgtype.UUID) error
 	ClearKoboShelfByName(ctx context.Context, arg ClearKoboShelfByNameParams) error
@@ -137,6 +137,7 @@ type Querier interface {
 	// Get all system config
 	GetAllSystemConfig(ctx context.Context) ([]SystemConfig, error)
 	GetAllSystemSettings(ctx context.Context) ([]GetAllSystemSettingsRow, error)
+	GetAllSystemSettingsFull(ctx context.Context) ([]SystemSettings, error)
 	GetAnnotationsForBook(ctx context.Context, arg GetAnnotationsForBookParams) ([]GetAnnotationsForBookRow, error)
 	GetBooksByTag(ctx context.Context, arg GetBooksByTagParams) ([]MediaItems, error)
 	// Get collection
@@ -278,6 +279,7 @@ type Querier interface {
 	GetSystemConfig(ctx context.Context, key string) (SystemConfig, error)
 	// System Settings queries
 	GetSystemSetting(ctx context.Context, settingKey string) (string, error)
+	GetSystemSettingFull(ctx context.Context, settingKey string) (SystemSettings, error)
 	GetSystemTimezone(ctx context.Context) (string, error)
 	GetTombstonedAnnotationsForBook(ctx context.Context, arg GetTombstonedAnnotationsForBookParams) ([]GetTombstonedAnnotationsForBookRow, error)
 	// Get universal progress for a book
@@ -428,6 +430,7 @@ type Querier interface {
 	UpsertDashboardPreferences(ctx context.Context, arg UpsertDashboardPreferencesParams) (UserDashboardPreferences, error)
 	UpsertPanelData(ctx context.Context, arg UpsertPanelDataParams) (PanelData, error)
 	UpsertReaderSettings(ctx context.Context, arg UpsertReaderSettingsParams) (ReaderSettings, error)
+	UpsertSystemSetting(ctx context.Context, arg UpsertSystemSettingParams) (SystemSettings, error)
 }
 
 var _ Querier = (*Queries)(nil)