Simplify meme preprocessing and upload failure handling - #396

Meme upload moderation imported image preprocessing through a Prefect ingestion flow, and batch failure handling repeated the same stop rule. This change puts watermarking behind one bytes-returning storage function used by both callers, unifies failure accounting, and removes the single-use upload-source wrapper. Upload source access now imports its focused repository module directly.

Behavior is preserved: JPEG output, non-image passthrough, invalid-image handling, five consecutive failures stopping a batch, reset on success, and uploader source reuse. Production code shrinks by 29 lines; no dependencies or migrations are added.

Validation:

• Three independent reviews: ponytail-audit, thermo-nuclear-code-quality-review, improve-codebase-architecture; no blockers after simplifications.
• Full suite: 601 passed, 2 skipped on Python 3.14 with disposable PostgreSQL/Redis.
• Ruff check/format, public-repo audit, Dockerfile.prod build passed.
• Synced with current production security updates before validation.