Add <meta name=description> to all public pages; improve page titles

- templates/public/head.php: emit <meta name="description"> when $metaDescription is set
- index.php: title → 'Posterg – Mémoires de l\'ERG'; description = site blurb
- tfe.php: title → '[Titre] – [Auteur] – Posterg'; description = synopsis excerpt (strip_tags, truncate 160)
- search.php: description = répertoire purpose blurb
- apropos.php: description = about-page blurb
- licence.php: description = licences blurb

Fixes WCAG 2.4.2 (Page Titled) for index.php and tfe.php.
All descriptions properly htmlspecialchars-escaped at render time.

TODO.md

@@ -534,7 +534,7 @@ Goal: rename the tables and column to the canonical M2M pattern (`tags`, `thesis
       `<form>` and no `<label>` for the input (only a placeholder). Add `aria-label="Recherche"` to
       the `<form>` element; also add `role="search"`, a visually-hidden `<label>` linked via `for`/`id`.
 
-- [ ] **No `<meta name="description">` on any public page** - all public pages omit the description
+- [x] **No `<meta name="description">` on any public page** - all public pages omit the description
       meta tag (the dead `templates/head.php` had `content=""`). Add per-page descriptions:
       site blurb for `index.php`, synopsis excerpt for `tfe.php`, page content intro for
       `apropos.php`/`licence.php`. Necessary for search indexing and link preview cards.
@@ -1100,7 +1100,7 @@ Current state: **zero ARIA attributes, zero skip links, zero focus-visible style
 
 #### 2.4.2 Page titled
 
-- [ ] **`index.php` `<title>` is just "Posterg"** - no description of the page content.
+- [x] **`index.php` `<title>` is just "Posterg"** - no description of the page content.
       Change to "Posterg - Mémoires de l'ERG" or similar. Each page title should be unique and
       descriptive first: "Répertoire - Posterg", "À Propos - Posterg" (already good), but
       `tfe.php` uses just the thesis title without author: add author - "[Titre] - [Auteur] - Posterg".