Fix outdated README and defer self-hosting docs to implementation

- Remove aspirational self-hosting section (docker-compose, config table,
  image storage) — will be written when features are actually implemented
- Fix build commands: mvn → ./mvnw, npm test → npm run test:unit
- Remove nonexistent Dockerfile from project structure tree
- Remove Testcontainers from prerequisites (not yet a dependency)
- Add README documentation ACs to US-13 (MAX_ACTIVE_EVENTS) and US-16
  (UNSPLASH_API_KEY + volume mount)

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

spec/userstories.md

@@ -320,6 +320,7 @@ The following terms are used consistently across all stories:
 - [ ] Only non-expired events count toward the limit; expired events awaiting cleanup are not counted
 - [ ] The limit is enforced server-side; it cannot be bypassed by the client
 - [ ] No personal data is logged when the limit is hit — only the rejection response is returned
+- [ ] The `MAX_ACTIVE_EVENTS` environment variable is documented in the README's self-hosting section (configuration table)
 
 **Dependencies:** US-1, T-4
 
@@ -386,7 +387,7 @@ The following terms are used consistently across all stories:
 - [ ] If the server has no Unsplash API key configured, the image search feature is unavailable — the option is simply not shown, no error
 - [ ] If the API key is removed from the config after images were already stored, existing images continue to render from disk; only the search/select UI becomes unavailable; the event page never breaks due to a missing API key
 - [ ] When the event expires and is deleted (US-12), the stored image file is deleted along with all other event data
-- [ ] The hoster must mount a persistent volume for the image storage directory (documented in README)
+- [ ] The `UNSPLASH_API_KEY` environment variable and the persistent volume requirement for image storage are documented in the README's self-hosting section (configuration table and storage notes)
 
 **Dependencies:** US-1, US-2, T-4