Postiz

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Build me the social scheduler I actually need instead of a hosted Postiz: a calendar, several platforms, and posts that go out once.

Read this first: Postiz is open source and self-hosting it is a real answer — the fee buys hosting and, more importantly, applications already approved for publishing. That approval is the wall this project hits, and it is administrative rather than technical. Read the next section before writing any code.

THE APPLICATION PROBLEM
- Every platform requires a developer application, and several require a review before it may post on a user's behalf. Some restrict posting to business accounts, some charge for the tier that allows it, and some have removed the capability entirely for small applications
- Check, for each platform you actually need, whether an independent developer can still get publishing permission today. Do that first. If the answer is no for the platform that matters to you, this project cannot succeed however good the code is
- Write what you find in the README, with dates, because it changes

STACK
- Node 20+ with Fastify, server-rendered HTML
- SQLite through better-sqlite3, WAL mode
- A worker for publishing, separate from the web process
- Caddy in front

THE DATA MODEL
- channels: id, platform, handle, display_name, avatar_path, credentials_encrypted, token_expires_at, refreshed_at, is_active, limits_json
- posts: id, body, media_ids_json, link, created_by, created_at, status, approval_state
- targets: id, post_id, channel_id, scheduled_for, status, attempts, client_key, external_id, external_url, published_at, error, platform_body — one row per channel, because the same words are not the same post everywhere
- media: id, path, sha256, kind, width, height, duration_ms, alt_text, bytes
- slots: id, channel_id, weekday, time_local, position — the recurring queue
- metrics: id, target_id, kind, value, collected_at — append-only, so a number is never overwritten
- Every instant UTC, with each channel's own zone kept for display

PUBLISHING WITHOUT DUPLICATES
- The worker claims a target with a lease, publishes, and writes the external identifier in the same transaction as the status change
- Idempotency: a platform that times out may still have published. Before retrying, look for the post — by a client-side key where the platform supports one, otherwise by searching the recent timeline for a match. A duplicate post is a public embarrassment and the failure this product must not have
- Retries with backoff for transient errors only, never for a validation failure
- Rate limits per channel, honouring the platform's own headers
- A failed target alerts by email with the platform's message verbatim

PER-PLATFORM LIMITS AS DATA
- Character count and how it is counted — a link may cost a fixed number of characters regardless of length, and an emoji may count as two
- Media count, accepted types, aspect ratios, file size, video length and codec
- Whether threads are supported, whether a first comment is possible, whether alt text is accepted
- All declared per platform in one table, and validated at scheduling time rather than at publish time. Finding out at 09:00 that a post is four characters too long is the thing to prevent

COMPOSING
- Write once, then per-platform overrides in tabs, each with a live preview at the size it will actually appear
- Threads where supported, as an ordered list
- Alt text on every image, with an option to refuse scheduling without it. It costs nothing and it is the difference between a considerate account and a careless one
- A link shortened through your own domain, so clicks are yours even when a platform will not tell you

THE CALENDAR AND THE QUEUE
- Slots per channel per weekday; adding to the queue fills the next free slot
- Drag to move, shuffle, pause a channel, and a holiday mode that pauses everything
- A month calendar across channels, and a list of the next fortnight
- Bulk import from a CSV for a campaign

APPROVALS
- Draft, needs approval, approved, scheduled, published, failed
- A reviewer sees the preview per platform and approves or returns with a note
- Nothing publishes from needs-approval by any path, ever

METRICS
- Collected on a schedule after publishing — an hour, a day, a week — and appended, never updated
- Absent where a platform does not provide them, and shown as absent rather than as zero
- Per post, per channel, per platform, computed at query time
- Link clicks from your own short domain, which is the one number nobody can take away

OPERATIONS
- .env: DATABASE_PATH, STORAGE_PATH, BASE_URL, ENCRYPTION_KEY, SESSION_SECRET, SMTP_URL, per-platform client ids and secrets
- Migrations on boot, each once
- Tokens refreshed under a lock so two workers cannot invalidate each other
- Nightly backup off the machine, restore script
- Health endpoint reporting the oldest due target and any channel with an expiring token

WHAT MATTERS MOST
The application review, then idempotent publishing. Confirm you can actually get permission to post before writing the scheduler, then build the lease and the duplicate check and pull the plug on the worker mid-publish to prove you get one post rather than none or two.

Give me the repository, one platform adapter implemented fully, migrations, .env.example, and a README with deploy steps behind Caddy and the current application situation for each platform.