Add Open Graph and Twitter Card meta-tags for link previews

Replace PathResourceResolver SPA fallback with SpaController that
injects OG/Twitter meta-tags into cached index.html template.
Event pages get event-specific tags (title, date, location),
all other pages get generic fete branding. Includes og-image.png
brand asset and forward-headers-strategy for proxy support.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

specs/012-link-preview/data-model.md

@@ -0,0 +1,83 @@
+# Data Model: Link Preview (Open Graph Meta-Tags)
+
+**Feature**: 012-link-preview | **Date**: 2026-03-09
+
+## Overview
+
+This feature does NOT introduce new database entities. It reads existing event data and projects it into HTML meta-tags. The "model" here is the meta-tag value object used during HTML generation.
+
+## Meta-Tag Value Objects
+
+### OpenGraphMeta
+
+Represents the set of Open Graph meta-tags to inject into the HTML response.
+
+| Field | Type | Source | Rules |
+|---|---|---|---|
+| `title` | String | Event title or "fete" | Max 70 chars, truncated with "..." |
+| `description` | String | Composed from event fields or generic text | Max 200 chars |
+| `url` | String | Canonical URL from request | Absolute URL |
+| `type` | String | Always "website" | Constant |
+| `siteName` | String | Always "fete" | Constant |
+| `image` | String | Static brand image URL | Absolute URL to `/og-image.png` |
+
+### TwitterCardMeta
+
+| Field | Type | Source | Rules |
+|---|---|---|---|
+| `card` | String | Always "summary" | Constant |
+| `title` | String | Same as OG title | Max 70 chars |
+| `description` | String | Same as OG description | Max 200 chars |
+
+## Data Flow
+
+```
+Request for /events/{token}
+    │
+    ▼
+LinkPreviewController
+    │
+    ├── Resolve event token → Event domain object (existing EventRepository)
+    │
+    ├── Build OpenGraphMeta from Event fields:
+    │     title     ← event.title (truncated)
+    │     description ← formatDescription(event.dateTime, event.timezone, event.location, event.description)
+    │     url       ← request base URL + /events/{token}
+    │     image     ← request base URL + /og-image.png
+    │
+    ├── Build TwitterCardMeta (mirrors OG values)
+    │
+    ├── Inject meta-tags into cached index.html template
+    │
+    └── Return modified HTML
+
+Request for / or /create (non-event pages)
+    │
+    ▼
+LinkPreviewController
+    │
+    ├── Build generic OpenGraphMeta:
+    │     title       ← "fete"
+    │     description ← "Privacy-focused event planning. Create and share events without accounts."
+    │     url         ← request URL
+    │     image       ← request base URL + /og-image.png
+    │
+    ├── Build generic TwitterCardMeta
+    │
+    ├── Inject meta-tags into cached index.html template
+    │
+    └── Return modified HTML
+```
+
+## Existing Entities Used (Read-Only)
+
+### Event (from `de.fete.domain.model.Event`)
+
+| Field | Used For |
+|---|---|
+| `title` | `og:title`, `twitter:title` |
+| `description` | Part of `og:description`, `twitter:description` |
+| `dateTime` | Part of `og:description` (formatted) |
+| `timezone` | Date formatting context |
+| `location` | Part of `og:description` |
+| `eventToken` | URL construction |