docs: unique filenames in changelog, help page, README and llms.txt

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BNhLunz586g6DeYMa7Qsd1

CHANGELOG.md

@@ -2,6 +2,42 @@
 
 All notable changes to Klarbild are documented here. Newest first.
 
+## 2026-08-21 — Unique, chronologically sortable filenames
+
+### Fixed
+- **Pictures overwrote each other at Picdrop.** Two badges of the same person in the
+  same format on the same day both came out as `2026-07-23_lea_the-frame.png`, so the
+  second upload silently replaced the first. Reported from real use — Till lost images
+  this way. The name now carries the time down to the second plus a short tag derived
+  from the item id:
+
+      2026-08-20_143207_lea-badge_the-frame_a3f9.jpg
+
+  Two pictures can no longer collide: the tag comes from the item id, which is unique.
+  A belt-and-braces loop appends `-2`, `-3` … if a name is somehow still taken —
+  deliberately a code loop rather than a UNIQUE constraint, because a rejected write
+  would fail an already-paid AI job at the very last step.
+
+### Added
+- **Backfill for the existing library** (Admin → Dateinamen). Preview first — it reports
+  how many names are currently duplicated — then apply. It re-stamps from `created_at`
+  and the item id while keeping the motif and format already chosen, so it is idempotent:
+  a second run changes nothing. Stored files are untouched; they live under their id,
+  not their name.
+- **Re-deliver everything** under the new name, for copies already sitting at Picdrop.
+  The old files stay where they are — deleting a customer's files automatically is the
+  wrong reflex; that stays a manual decision.
+
+### Changed
+- Timestamps use **Europe/Berlin** (override with `KLARBILD_TZ`) rather than UTC. A
+  picture taken at 00:30 should not be dated to the previous day.
+- Naming moved into `src/lib/naming.ts` — dependency-free, like `printlayout.ts`, so the
+  rules are testable and identical in every path (jobs, Telegram, MCP, backfill).
+  The generator strips its own stamp when a result is fed back in as a source, so
+  re-processing cannot accumulate timestamps.
+- 17 new tests (56 total), including the reported bug as an explicit regression test and
+  a check that lexicographic sorting equals chronological sorting across a DST boundary.
+
 ## 2026-08-20 — Oversize sheets, duplicate images, free labelling, new layout
 
 ### Added