feat: add insights pages — stats dashboard, plot of the day, similar specs

New features that leverage the database more heavily:

- Stats page (/stats): library quality/LOC histograms, coverage dot matrix,
  top rated implementations, tag cloud, timeline, plausible link
- Plot of the Day: daily featured high-quality impl on homepage, dismissable
- Similar specifications/implementations: tag-based Jaccard similarity on
  spec pages (spec tags) and impl pages (spec + impl tags), with hover
  highlighting of shared tags
- Lazy code loading: code field deferred in DB model (~25MB saved per
  get_all), new /specs/{id}/{lib}/code endpoint, frontend prefetch
- Collapsible spec tabs with always-visible tags and metadata footer
- New insights API router with /dashboard, /plot-of-the-day, /related
- Updated sitemap, plausible docs, API docs

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: MarkusNeusinger

api/main.py

@@ -27,6 +27,7 @@
     debug_router,
     download_router,
     health_router,
+    insights_router,
     libraries_router,
     og_images_router,
     plots_router,
@@ -128,6 +129,9 @@ async def add_cache_headers(request: Request, call_next):
     # Individual spec details (5 min cache, 1h stale-while-revalidate)
     elif path.startswith("/specs/"):
         response.headers["Cache-Control"] = "public, max-age=300, stale-while-revalidate=3600"
+    # Insights endpoints (5 min cache, 1h stale-while-revalidate)
+    elif path.startswith("/insights/"):
+        response.headers["Cache-Control"] = "public, max-age=300, stale-while-revalidate=3600"
 
     return response
 
@@ -141,6 +145,7 @@ async def add_cache_headers(request: Request, call_next):
 app.include_router(specs_router)
 app.include_router(libraries_router)
 app.include_router(plots_router)
+app.include_router(insights_router)
 app.include_router(download_router)
 app.include_router(seo_router)
 app.include_router(og_images_router)

api/routers/__init__.py

@@ -3,6 +3,7 @@
 from api.routers.debug import router as debug_router
 from api.routers.download import router as download_router
 from api.routers.health import router as health_router
+from api.routers.insights import router as insights_router
 from api.routers.libraries import router as libraries_router
 from api.routers.og_images import router as og_images_router
 from api.routers.plots import router as plots_router
@@ -16,6 +17,7 @@
     "debug_router",
     "download_router",
     "health_router",
+    "insights_router",
     "libraries_router",
     "og_images_router",
     "plots_router",