backend(infra): shared config helper, startup recovery, periodic maintenance

Foundations for the v0.16 features. No new endpoints here — those land in
the next commit on top of these.

- migrations 008 + 009: commit the load-bearing compression_status column
  that was uncommitted on disk; add 009_feature_toggles seeding the master
  + per-endpoint rate-limit switches, the master + per-area quota switches,
  and the admin-editable privacy_note.
- services/config.rs (new): get_str / get_i64 / get_usize / get_f64 / get_bool
  consolidating the scattered helpers that lived in three handlers.
- services/maintenance.rs (new):
  - startup_recovery() — resets compression_status='processing' and
    export_job.status='running' rows orphaned by a previous crashed
    instance, so users never see permanent "Wird vorbereitet…" spinners.
  - spawn_periodic_tasks() — hourly cleanup of expired sessions (rows
    were never pruned) + rate-limiter HashMap pruning (windows kept one
    entry per IP forever).
- services/jobs.rs (new sketch): BackgroundJob trait + JobContext for
  future jobs to plug into the same progress + SSE pipeline as
  compression/export. Not wired yet — codifies the convention.
- services/compression.rs: 120s hard timeout + kill_on_drop on ffmpeg
  so a malformed video can't hang and leak a worker semaphore permit.
- services/rate_limiter.rs: new prune() called from the periodic task.
- state.rs: SseEvent::new() constructor so event-type strings stay
  consistent instead of being typed inline at every emit site.
- models/user.rs: UserRole::as_str() for /me/context serialization.
- models/upload.rs: soft_delete() now runs in a transaction and
  decrements the uploader's total_upload_bytes (GREATEST(0, …) guard) —
  fixes a quota drift where deleting reclaimed no quota.
- Cargo.toml + Cargo.lock: add `infer = "0.15"` (multipart MIME sniffing
  used by the upload handler).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/migrations/008_compression_status.up.sql

@@ -0,0 +1,6 @@
+-- Add compression_status to track media processing state
+ALTER TABLE upload ADD COLUMN compression_status TEXT NOT NULL DEFAULT 'pending';
+
+-- Values: 'pending', 'processing', 'done', 'failed'
+-- Add comment to document the field
+COMMENT ON COLUMN upload.compression_status IS 'Tracks media compression/preview generation: pending -> processing -> (done or failed)';