Feat/preview missing status

[nledez]

Problem

• renormalize-movie-preview-files failures left PreviewFile.status stuck on processing, so Kitsu kept showing affected previews as processing forever even though the
  renormalize pass had given up.
• The single broken status conflated two very different cases: a movie that exists but cannot be normalized (encode/ffmpeg error, transient storage hiccup) versus a movie
  whose source binary is simply gone from object storage. Operators could not target the irrecoverable rows without retrying the recoverable ones too.
• sync-full-files re-downloaded every preview every time, including rows known to be unusable, wasting bandwidth and time on previews that will never play.

Solution

• renormalize-movie-preview-files now marks failed rows on exit: source-binary-missing → new missing status, other failures → broken. The 3-preview reproducer that prompted
  this no longer stays as processing.
• New internal status missing added to PreviewFile.STATUSES. No DB migration needed — ChoiceType is enforced at ORM level only.
• EXPOSE_MISSING config flag (env var, default false) gates the API exposure of the new status. With the default, missing is folded back to broken in serialize(), present()
  and present_minimal() so existing API/Kitsu clients see no change. This flag is intentionally temporary: it gives us time to (1) announce the new status publicly, (2) ship the
  matching Kitsu UI/UX changes, and (3) let third-party clients adopt. Once Kitsu speaks missing natively, EXPOSE_MISSING will be removed and missing will be surfaced
  unconditionally.
• renormalize-movie-preview-files gains a --all-missing flag (sibling of --all-broken / --all-processing); --all-broken stays broken-only, --all-missing is the new operator
  escape hatch to retry rows whose source has since been restored.
• sync-full-files gains --skip-broken and --skip-missing flags (default off, so behavior is unchanged) to let operators exclude unusable rows from large pulls.
• New service helper preview_files_service.set_preview_file_as_missing(); internal queries that filter ("broken", "processing") now include "missing" so it is treated the same
  as broken for "needs attention" purposes.
• Tests added for the missing→broken serialization remap (both EXPOSE_MISSING values) and for the --all-missing filter behavior.

[frankrousseau]

Why do you add this flag? It's ok to exposte missing to everyone.