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/plan.md

@@ -0,0 +1,83 @@
+# Implementation Plan: Link Preview (Open Graph Meta-Tags)
+
+**Branch**: `012-link-preview` | **Date**: 2026-03-09 | **Spec**: `specs/012-link-preview/spec.md`
+**Input**: Feature specification from `/specs/012-link-preview/spec.md`
+
+## Summary
+
+Inject Open Graph and Twitter Card meta-tags into the server-rendered HTML so that shared event links display rich preview cards in messengers and on social media. The Spring Boot backend replaces its current static SPA fallback with a controller that dynamically injects event-specific or generic meta-tags into the cached `index.html` template before serving it.
+
+## Technical Context
+
+**Language/Version**: Java 25 (backend), TypeScript 5.9 (frontend — minimal changes)
+**Primary Dependencies**: Spring Boot 3.5.x (existing), Vue 3 (existing) — no new dependencies
+**Storage**: PostgreSQL via JPA (existing event data, read-only access)
+**Testing**: JUnit 5 + Spring MockMvc (backend), Playwright (E2E)
+**Target Platform**: Self-hosted Docker container (Linux)
+**Project Type**: Web application (SPA + REST API)
+**Performance Goals**: N/A — meta-tag injection adds negligible overhead (<1ms string replacement)
+**Constraints**: Meta-tags MUST be in initial HTML response (no client-side JS injection). No external services or CDNs.
+**Scale/Scope**: Affects all HTML page responses. No new database tables or API endpoints.
+
+## Constitution Check
+
+*GATE: Must pass before Phase 0 research. Re-check after Phase 1 design.*
+
+| Principle | Status | Notes |
+|---|---|---|
+| I. Privacy by Design | ✅ PASS | No tracking, analytics, or external services. Meta-tags contain only public event info (title, date, location). No PII exposed. `og:image` is a self-hosted static asset. |
+| II. Test-Driven Methodology | ✅ PLAN | Unit tests for meta-tag generation, integration tests for controller, E2E tests for full HTML verification. TDD enforced. |
+| III. API-First Development | ✅ N/A | No new API endpoints. This feature modifies HTML serving, not the REST API. Existing OpenAPI spec unchanged. |
+| IV. Simplicity & Quality | ✅ PASS | Simple string replacement in cached HTML template. No SSR framework, no prerendering service, no user-agent sniffing. Minimal moving parts. |
+| V. Dependency Discipline | ✅ PASS | Zero new dependencies. Uses only Spring Boot's existing capabilities. |
+| VI. Accessibility | ✅ N/A | Meta-tags are invisible to users. No UI changes. |
+
+**Gate result: PASS** — No violations. No complexity tracking needed.
+
+## Project Structure
+
+### Documentation (this feature)
+
+```text
+specs/012-link-preview/
+├── plan.md              # This file
+├── research.md          # Phase 0 output — technical decisions
+├── data-model.md        # Phase 1 output — meta-tag value objects
+├── quickstart.md        # Phase 1 output — implementation guide
+├── contracts/
+│   └── html-meta-tags.md  # Phase 1 output — meta-tag HTML contract
+└── tasks.md             # Phase 2 output (created by /speckit.tasks)
+```
+
+### Source Code (repository root)
+
+```text
+backend/
+├── src/main/java/de/fete/
+│   ├── adapter/in/web/
+│   │   └── SpaController.java            # NEW — serves index.html with injected meta-tags
+│   ├── application/
+│   │   └── OpenGraphService.java          # NEW — composes meta-tag values from event data
+│   └── config/
+│       └── WebConfig.java                 # MODIFIED — remove PathResourceResolver SPA fallback
+├── src/main/resources/
+│   └── application.properties             # MODIFIED — add forward-headers-strategy
+└── src/test/java/de/fete/
+    ├── adapter/in/web/
+    │   └── SpaControllerTest.java         # NEW — integration tests
+    └── application/
+        └── OpenGraphServiceTest.java      # NEW — unit tests
+
+frontend/
+├── index.html                             # MODIFIED — add <!-- OG_META_TAGS --> placeholder
+├── public/
+│   └── og-image.png                       # NEW — brand image for og:image (1200x630)
+└── e2e/
+    └── link-preview.spec.ts               # NEW — E2E tests
+```
+
+**Structure Decision**: Web application structure (existing). Backend changes in adapter/web and application layers. Frontend changes minimal (HTML placeholder + static asset).
+
+## Complexity Tracking
+
+> No violations — section intentionally empty.