feat: Reddit channel

[paulocastellano]

Reddit channel

Adds Reddit as a publishing channel. Users connect their own Reddit account via OAuth and submit to one or more subreddits per post — following the existing channel architecture (Discord as the reference).

What it does

• OAuth connect/callback — custom Socialite provider (Basic-auth token exchange, duration=permanent, mandatory User-Agent), token verify + refresh in ConnectionVerifier.
• Self + link posts to one or many subreddits in a single post (meta.subreddits[]), paced for Reddit's rate limit, with per-subreddit partial-failure handling (succeeded submissions are persisted; the failing subreddit is surfaced).
• Single-image posts via Reddit's asset-lease flow (/api/media/asset → S3 → <Location> from XML).
• Live subreddit search + per-subreddit restrictions (submission type, image allowance) and flair (including required-flair enforcement) via authenticated, workspace-scoped lookup endpoints.
• Post metrics — upvotes + comments per post (summed across subreddits) and account karma. (Postiz does not expose Reddit metrics.)
• Composer UI — a multi-subreddit settings panel (search, title, type, URL, flair, NSFW/spoiler) + a Reddit post preview, plus the connectable-channel registration and logo.
• Validation lives only in PostPlatformMetaRules (web + public API + MCP all spread it), including required-on-publish (≥1 subreddit, title, URL for link posts, required flair).
• i18n in en / es / pt-BR.

Deferred (follow-up)

• Video & gallery posts. Reddit video requires a video_poster_url, and there is no frame-extraction/poster pipeline in the app (no ffmpeg). The UI and validation only offer self/link/image, so there's no confusing publish-time failure — video/gallery simply aren't advertised yet.

Testing

• Full suite green: 2145 passed, 2 skipped.
• 47 Reddit-specific tests across enums, Socialite provider, OAuth, token refresh, client (search/restrictions/flair/info), publisher (self/link/multi-subreddit/partial-failure/image), metrics, lookup endpoints, and per-platform meta (API + MCP).

Notes

• Reddit posts use the connected user's own OAuth token (not a shared bot), so a user can only post where their account can; lookup endpoints enforce workspace ownership.
• All third-party hosts come from config/trypost.php (platforms.reddit); nothing hardcoded.