feat: insights pages — stats, plot of the day, similar specs (#5233)

## Summary

- **Stats Dashboard** (`/stats`): Library quality/LOC histograms,
coverage dot matrix, top rated implementations, tag cloud, timeline,
Plausible analytics link
- **Plot of the Day**: Daily featured high-quality implementation on
homepage (dismissable, sessionStorage persisted)
- **Similar Specifications/Implementations**: Tag-based Jaccard
similarity — spec pages use spec tags, impl pages use spec + impl tags,
with hover-highlighting of shared tags
- **Deferred code loading**: Heavy DB fields (`code`,
`review_criteria_checklist`, `review_image_description`, `tested`)
deferred in model (~25MB saved per `get_all()`), new lightweight
`/specs/{id}/{lib}/code` endpoint, frontend prefetch on impl page open
- **Collapsible spec tabs** with always-visible tags and metadata footer
showing updated dates
- **New insights API router** (`/insights/dashboard`,
`/insights/plot-of-the-day`, `/insights/related/{spec_id}`)
- Updated sitemap, Plausible docs, API docs, footer links

Closes #5228
Closes #5229
Closes #5230

## Test plan

- [ ] Verify `/stats` page loads with all sections (counters, library
histograms, coverage matrix, top rated, timeline, tags)
- [ ] Verify Plot of the Day appears on homepage when no filters active,
dismiss persists across navigation
- [ ] Verify similar specifications on spec overview page (spec tags
only, links to specs)
- [ ] Verify similar implementations on impl detail page (spec + impl
tags, links to impls, same library preferred)
- [ ] Verify tag hover highlighting works between similar cards and tag
chips
- [ ] Verify code tab loads lazily (check network tab — no code in
`/specs/{id}` response)
- [ ] Verify all tabs collapse/expand on click including re-click to
close
- [ ] Run `uv run pytest tests/unit` — 1068 tests pass
- [ ] Run `yarn lint && yarn build` — 0 errors

🤖 Generated with [Claude Code](https://claude.ai/claude-code)

---------

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",