fix(gamedata): planets refetched in one cached batch instead of one request per planet

[jplacht]

What and why

When planets expired, the frontend refetched every planet with its own GET /data/planet/<id>/ at once (bursts of up to 117 requests per client in production), and POST /data/planets/multiple/ was not cached. This serves planets/multiple from the same per-planet cache entries as the detail endpoint, so the frontend can refetch expired planets in one request.

Changes

• planets/multiple served from the per-planet PLANET cache entries it shares with GET /data/planet/<id>/ (new CacheManager.respond_many)
• missing planets built in one queryset and cached per planet; unknown ids omitted, never cached
• ETag over the member ETags, If-None-Match → 304, Cache-Control: public, max-age=60
• at most 200 ids per request (400 otherwise)
• removed the unused GamePlanetSearchService.search_by_planet_natural_id

Deviation: misses skip the per-key stampede lock (they're built in one prefetching query, so the lock would only serialise encoding).

How it was verified

QA: PASS in one round (code review against all 6 acceptance criteria, full walk in the running app).

• uv run ruff check, ruff format --check, ty check, pytest: pass
• new tests: same bodies as the detail endpoint in request order, repeat request runs 0 queries, only new planets built, new data after invalidation, 304 on the current ETag, 201 ids → 400
• running app: ETag, public, max-age=60, 304 on repeat, 400 on 201 ids; the plan editor still loads its planet

Deploy order

Deploy backend first; it is backward compatible (same response shape, the current frontend sends far fewer than 200 ids). Paired frontend PR: PRUNplanner/frontend#534

🤖 Generated with Claude Code

[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

Results:
0 new issues

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🟢 Metrics 21 complexity · 0 duplication

Metric	Results
Complexity	21
Duplication	0

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🟢 Coverage 97.87% diff coverage · +0.01% coverage variation

Metric	Results
Coverage variation	✅ +0.01% coverage variation (-1.00%)
Diff coverage	✅ 97.87% diff coverage

View coverage diff in Codacy

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (927be54)	4600	4391	95.46%
Head commit (a82aafe)	4633 (+33)	4423 (+32)	95.47% (+0.01%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#64)	47	46	97.87%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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