Add anki-sync-server service — self-hosted Anki flashcard sync

Docker-based sync backend for the Anki app (afrima/anki-sync-server, the
official Rust sync server). Supports multiple independent accounts per
instance, the repo's multi-instance pattern, port collision avoidance,
Caddy wiring, and update/fresh/cancel reinstall detection.

No Authelia gate — this is a raw sync API the Anki client talks to, not
a browser session, so a forward_auth portal in front of it would just
break every sync request; SYNC_USER1/SYNC_USER2/... is its own auth
boundary.

Companion services/anki-sync-server.md covers client setup (Desktop,
AnkiDroid, AnkiMobile) and the Quizlet import/export walkthrough.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014DyceEVVeQ33EeS6C1PDv5

services/anki-sync-server.md

@@ -0,0 +1,82 @@
+## Client setup — pointing Anki at this server instead of AnkiWeb
+
+Every client below needs the **Sync URL** and one of the **accounts** shown
+higher up in this README. Do this on every device you want synced — a client
+still pointed at AnkiWeb won't see collections synced here, and vice versa.
+
+### Anki Desktop (2.1.66 and newer)
+1. **Preferences → Network**
+2. Tick **"Self-hosted sync server"**
+3. Paste the Sync URL into the field that appears
+4. **Sync → log in** with one of the accounts above
+
+### Anki Desktop (older than 2.1.66)
+There's no GUI field yet — set an environment variable before launching Anki
+instead, then sync normally:
+```bash
+# Linux/macOS
+export SYNC_ENDPOINT="https://your-sync-url/"
+anki
+
+# Windows (Command Prompt)
+set SYNC_ENDPOINT=https://your-sync-url/
+anki.exe
+```
+Upgrading Anki to 2.1.66+ is the easier long-term fix — do that if you're
+setting this up for anyone who isn't comfortable with environment variables.
+
+### AnkiDroid
+**Settings → Advanced → Custom sync server**, then enter the Sync URL and
+log in with one of the accounts above (AnkiDroid 2.16+; update the app if
+this option isn't there).
+
+### AnkiMobile (iOS)
+**Settings → Advanced → Custom Sync Server**, same as AnkiDroid — enter the
+Sync URL and log in.
+
+### First sync on each device
+The very first sync from a device that already has a local collection will
+ask whether to upload local data or download from the server — pick upload
+from whichever device has your real collection, and download on every other
+device, or you'll end up with two different collections that never merge.
+
+## Importing your existing Quizlet sets
+
+This server only handles syncing already-existing Anki collections — it
+doesn't import anything itself. Quizlet import happens once, locally, in the
+Anki desktop app, before your first sync:
+
+1. **In Quizlet:** open the set → **Export** → choose the plain-text /
+   tab-separated format (Quizlet's export dialog lets you pick the delimiter
+   between term and definition, and between rows — tab and newline are the
+   Anki-friendly defaults) → copy the exported text or download it as a
+   `.txt`/`.csv` file.
+2. **In Anki Desktop:** **File → Import**, pick the file (or paste the text
+   into a `.txt` file first if you copied it to the clipboard).
+3. Map the two columns to **Front** and **Back** in the import dialog, pick
+   or create the deck and note type, and import.
+4. For **math facts or other simple front/back cards**, the Basic note type
+   is enough. For **more complex cards** (extra example fields, images,
+   audio, cloze deletions), switch the note type in the import dialog to a
+   template with more fields, or convert cards afterward — Anki's own
+   built-in note types (Basic, Basic (and reversed card), Cloze) cover most
+   of what Quizlet's own card types can do.
+5. Sync from this device once the import looks right, so the imported deck
+   becomes the copy every other device downloads.
+
+### Exporting back out (Anki → Quizlet or anywhere else)
+**File → Export**, choose "Notes in Plain Text" and pick the deck — this
+produces the same tab-separated format Quizlet's own import expects, so the
+round trip works in both directions.
+
+## Why spaced repetition here actually reschedules failed cards
+
+Anki's scheduler (FSRS, the default since recent Anki versions) tracks a
+per-card memory-strength estimate and schedules the next review right before
+you'd be expected to forget it. Answering "Again" on a card doesn't just
+requeue it for later the same session — it lowers that card's estimated
+strength, which shortens every subsequent interval for it until you've
+proven you know it again, so a card you keep failing gets shown far more
+often than one you consistently get right. This is scheduling logic inside
+the Anki client itself; this sync server only stores and syncs the resulting
+review history, it doesn't change how reviews are scheduled.