perf: drop the cache layers that measured at zero

Ablation (PHASE2-FINDINGS.md) attributed the memory win to the
serverExternalPackages line and the integrations barrel diet. These three
measured at exactly zero and were most of the file churn, so they go:

next-cache-handler: orphaned. cacheHandlers and cacheComponents are both
already gone from next.config.ts and nothing imported the package — it
was reachable only through a stale dependency entry.

cache-tags / cache-invalidation: there is no "use cache", cacheTag,
cacheLife or unstable_cache anywhere in app source, so revalidateTag was
firing on tags nothing registers, with no handler to receive them, inside
a catch that swallowed the resulting out-of-scope error. Removing the
calls also removed the SELECTs that existed only to supply a board name
or member list to them, so board.ts, group.ts, user.ts, serverSettings.ts,
section-router.ts and change-search-preferences.ts return to the
merge-base exactly. The live in-process integration data cache is
untouched.

Redis 6 -> 2 client consolidation: no measurable effect at idle (139.3 vs
139.2 MiB) or under load. Reverted, but the pubSub:last:* TTL is kept —
that one is an unbounded-growth fix, not a consolidation.

23 files, -883 lines. Diff vs merge-base: 100 files/3336 insertions ->
82 files/2484. pnpm test 144/144 files, 1795 tests, 0 failures;
typecheck and lint clean; app and Docker image build.

apps/nextjs/package.json

@@ -37,7 +37,6 @@
     "@homarr/integrations": "workspace:^0.1.0",
     "@homarr/modals": "workspace:^0.1.0",
     "@homarr/modals-collection": "workspace:^0.1.0",
-    "@homarr/next-cache-handler": "workspace:^0.1.0",
     "@homarr/notifications": "workspace:^0.1.0",
     "@homarr/old-import": "workspace:^0.1.0",
     "@homarr/old-schema": "workspace:^0.1.0",