docs: add admin tag management UI task to TODO

TODO.md

@@ -100,7 +100,99 @@ Goal: rename the tables and column to the canonical M2M pattern (`tags`, `thesis
 - [ ] `public/tfe.php`: `$data['keywords']` → `$data['tags']` (view column rename)
 - [ ] `templates/search-bar.php` (if applicable): update any hardcoded `keyword` param refs
 
-### 5 — Tests
+### 5 — Admin tag management UI (`/admin/tags.php`)
+
+The goal is a dedicated page for viewing, renaming, merging, and deleting tags, with
+full referential-integrity awareness (no orphan `thesis_tags` rows, no broken search
+results).
+
+#### 5a — `src/Database.php` — new tag-management methods
+
+- [ ] `getAllTagsWithCount(): array` — return all tags with a `thesis_count` column:
+      ```sql
+      SELECT t.id, t.name,
+             COUNT(tt.thesis_id) AS thesis_count
+      FROM tags t
+      LEFT JOIN thesis_tags tt ON t.id = tt.tag_id
+      GROUP BY t.id ORDER BY t.name
+      ```
+- [ ] `renameTag(int $id, string $newName): void` — UPDATE `tags SET name = ? WHERE id = ?`;
+      unique-constraint violation must be caught and re-thrown as a user-readable
+      `InvalidArgumentException`
+- [ ] `mergeTag(int $sourceId, int $targetId): void` — within a transaction:
+      1. INSERT OR IGNORE into `thesis_tags(tag_id, thesis_id)` for every row in
+         `thesis_tags WHERE tag_id = $sourceId` (re-point to target, skip dupes)
+      2. DELETE FROM `thesis_tags WHERE tag_id = $sourceId`
+      3. DELETE FROM `tags WHERE id = $sourceId`
+- [ ] `deleteTag(int $id): void` — within a transaction:
+      DELETE FROM `thesis_tags WHERE tag_id = ?` first (FK cascade may handle this
+      after the schema migration, but an explicit delete is safer), then
+      DELETE FROM `tags WHERE id = ?`; reject with exception if `thesis_count > 0`
+      and the caller did not pass `$force = true`
+
+#### 5b — `public/admin/tags.php` — list + inline-edit view
+
+- [ ] Auth guard: `AdminAuth::requireLogin()` at top; CSRF token in session
+- [ ] Page title: `"Gestion des mots-clés"` (reuses `$pageTitle` for `head.php`)
+- [ ] Load `getAllTagsWithCount()` for display
+- [ ] Table columns: **Mot-clé** (editable inline) · **Nb de TFE** · **Actions**
+- [ ] Each row has:
+  - An inline `<form>` (POST to `actions/tag.php`, action=`rename`) pre-filled with
+    the current tag name, so the admin can edit in-place and submit per row
+  - A **Fusionner →** button that reveals a `<select>` of other tags and a confirm
+    submit (POST action=`merge`, fields: `source_id`, `target_id`)
+  - A **Supprimer** button — disabled / greyed if `thesis_count > 0`; otherwise
+    submits POST action=`delete`, field: `tag_id`; requires JS `confirm()` before
+    submit
+- [ ] Flash success/error messages from `$_SESSION['success']` / `$_SESSION['error']`
+      (same pattern as `index.php`)
+- [ ] Empty state: "Aucun mot-clé enregistré." if table is empty
+- [ ] Stats bar (reuse `.admin-stats` CSS): total tag count, total TFE with ≥1 tag,
+      total tags with 0 TFE (orphan count)
+
+#### 5c — `public/admin/actions/tag.php` — POST action handler
+
+- [ ] Auth guard + CSRF check (same pattern as `publish.php`)
+- [ ] Route on `$_POST['action']`:
+  - `'rename'`: validate `tag_id` (int > 0) and `name` (non-empty, max 100 chars,
+    trimmed); call `renameTag()`; set `$_SESSION['success']`
+  - `'merge'`: validate `source_id` and `target_id` are distinct positive ints; call
+    `mergeTag()`; set success message including both tag names
+  - `'delete'`: validate `tag_id`; call `deleteTag()`; set success message
+- [ ] On any exception: set `$_SESSION['error']`
+- [ ] Regenerate CSRF token after every action
+- [ ] Redirect back to `/admin/tags.php` in all cases
+
+#### 5d — Nav & routing
+
+- [ ] `templates/admin/head.php`: add nav link
+      `<a href="/admin/tags.php" …>Mots-clés</a>` between "Ajouter un TFE" and
+      "Importer une liste de TFE"; apply `active` class on `tags.php`
+- [ ] `nginx/posterg.conf` (or equivalent): if using try_files / location blocks,
+      confirm `/admin/tags.php` and `/admin/actions/tag.php` are reachable (likely
+      already covered by the existing `*.php` rule, but verify)
+
+#### 5e — Propagation safety checklist
+
+These must remain unbroken after the tag-management UI is live:
+
+- [ ] Renaming a tag updates `tags.name`; `v_theses_full` reads from `tags` directly,
+      so `GROUP_CONCAT` output updates automatically — no view rebuild needed
+- [ ] Renaming a tag does **not** affect `thesis_tags` rows (only the name column
+      changes); search results remain intact
+- [ ] Merging tag A→B: all theses that had tag A now appear under tag B; the
+      `EXISTS` subquery in `buildSearchConditions` finds them via `thesis_tags`
+- [ ] Deleting a tag with `thesis_count > 0` is blocked by default (explicit
+      `$force` parameter required); the UI never shows a delete button for in-use
+      tags — double protection
+- [ ] `edit.php` keyword field renders the current CSV of tag names from
+      `$thesis['keywords']` (view column); after a rename the edit page will show
+      the new name on next load — no extra work needed
+- [ ] The public `search.php` keyword filter (`$_GET['tag']`) uses tag names; after a
+      rename, any bookmarked URL with the old name will return 0 results (expected
+      behaviour — no redirect needed, but note it in a code comment)
+
+### 6 — Tests
 
 - [ ] `tests/Unit/DatabaseTest.php`: add test for `findOrCreateTag` round-trip
 - [ ] `tests/Integration/SearchTest.php`: add test for tag-filter search using the new