feat(heatmap): Add outings map density endpoints (heatmap + tracks)

[Nayor]

Summary

• New GET /outings/map/heatmap endpoint: PostGIS grid aggregation (ST_SnapToGrid) of outings for a bbox, filterable by activity, meant to feed a density heatmap at low/medium map zoom.
• New GET /outings/map/tracks endpoint: individual outing tracks (geom_detail) intersecting a bbox, filterable by activity, capped at 100 results with an explicit truncated flag rather than a silent partial list — meant to feed a vector layer once the map is zoomed in enough for the outing count to stay bounded.
• Both query PostGIS directly rather than Elasticsearch, since geom_detail tracks only exist there and ST_Intersects on the real track is more correct than the existing centroid-only bbox= filter.
• Reuses the existing dogpile/Redis caching idiom (new cache_outing_map region).

Related: c2corg/c2c_ui#4573 (frontend counterpart), c2corg/c2c_ui#4573 has the community forum context behind this feature.

Test plan

• New unit tests in c2corg_api/tests/views/test_outing_map.py (bbox/zoom/activity validation, heatmap bucket counts, tracks list + activity filter + truncation) — all passing.
• Full existing test suite run for regressions — pre-existing failures unrelated to this change (a collections.Mapping Python 3.12 incompatibility already tracked by the separate python-3.12-migration branch); none touch the new files.
• Manual curl smoke test against a dev instance recommended before merge.

🤖 Generated with Claude Code

[codacy-production]

Not up to standards ⛔

🔴 Issues 21 minor

Alerts:
⚠ 21 issues (≤ 0 issues of at least minor severity)

Results:
21 new issues

Category	Results
Documentation	21 minor

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🟢 Metrics 50 complexity · 2 duplication

Metric	Results
Complexity	50
Duplication	2

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🟢 Coverage 96.86% diff coverage · +0.04% coverage variation

Metric	Results
Coverage variation	✅ +0.04% coverage variation
Diff coverage	✅ 96.86% diff coverage

View coverage diff in Codacy

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (9e1cdfb)	21972	20303	92.40%
Head commit (7a4e43f)	22163 (+191)	20488 (+185)	92.44% (+0.04%)

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 (#2231)	191	185	96.86%

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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