refactor: extract edit.php POST handler to actions/edit.php

edit.php was a 530-line file mixing form display, POST handling, file
uploads, and reference-data loading. This refactor splits it along the
same action-file pattern already used by formulaire.php, tag.php, and
page.php.

Changes:
- public/admin/actions/edit.php (new): standalone POST handler; auth
  guard, CSRF check, transaction, redirect with session flash messages
- public/admin/edit.php: display-only; reads edit_success/edit_error
  flash keys from session; form action points to actions/edit.php via
  a hidden thesis_id field instead of a query-string self-post
- src/Database.php: four new methods to remove all raw PDO from both
  files:
    - updateThesis(int, array): void  — UPDATE theses core fields
    - setThesisAuthors(int, array): void  — delete-then-reinsert authors
    - getThesisLanguageIds(int): array — SELECT language_id for form
    - getThesisFormatIds(int): array   — SELECT format_id for form

TODO.md

@@ -423,10 +423,11 @@ Goal: rename the tables and column to the canonical M2M pattern (`tags`, `thesis
 
 ### C — Code organisation / maintainability
 
-- [ ] **`edit.php` does too much** — 530 lines combining form display, POST handling, file upload,
-      and all reference-data loading in one file. Extract the POST handler to
-      `public/admin/actions/edit.php` (matching the pattern already used by `formulaire.php`,
-      `tag.php`, `page.php`, etc.).
+- [x] **`edit.php` does too much** — POST handler extracted to `public/admin/actions/edit.php`;
+      `edit.php` is now display-only (loads data, renders form, reads flash messages from session).
+      Added `Database::updateThesis()`, `Database::setThesisAuthors()`,
+      `Database::getThesisLanguageIds()`, `Database::getThesisFormatIds()` to remove all raw PDO
+      from both files. Matches the pattern of `formulaire.php`, `tag.php`, `page.php`.
 
 - [x] **`formulaire.php` duplicates banner-upload logic verbatim from `edit.php`** — extracted to
       `Database::handleBannerUpload(int $thesisId, ?array $uploadedFile): ?string`; both action