feat(api): status endpoint, fort scan limit status, availability enrichment

[jfberry]

Golbat-side prerequisites for the Diadem fort API migration (ccev/diadem#174 — the consumer design this serves).

Includes #392 as a cherry-pick with attribution preserved (thanks @ccev) — closes #392.

Deliverables

1. GET /api/status — secret-gated but not fort_in_memory-gated: reports enabled optional features and the server's scan caps so consumers can detect capabilities (and clamp their request limits) without probing gated endpoints:

   {
     "features": { "fort_in_memory": true },
     "limits": { "max_pokemon_results": 3000, "max_fort_results": 4000 }
   }

2. limit_reached on the fort scan responses (gym/pokestop/station/combined) — straight mirror of feat: expose v3 pokemon scan limit status #392: computed from the pre-collection key count against the effective limit (fortScanLimit = request limit clamped by tuning.max_fort_results).

3. Availability enrichment — temp_evolution_id on gym raid availability (mega/primal bosses were indistinguishable from base forms) and ranking_standard on showcase availability. Distinct evolutions now yield distinct entries; both fields thread through FortLookup → availability keys → readers.

4. Fix: pokestop first_seen_timestamp was int16 on both the model and ApiPokestopResult — a unix timestamp wraps int16, truncating DB reads and API output (gym's field was already int64). Note while fixing: nothing in Golbat currently assigns this field for pokestops (existing DB values round-trip; new stops insert 0) — populating it on first insert is a separate follow-up.

Verification

• go build -tags go_json ./...
• go test -tags go_json ./decoder/ . (new: fort limit helper tests, /api/status route tests incl. 401 + not-503-gated assertions, availability enrichment tests incl. mega-vs-base distinctness)
• go test -tags go_json -race -run 'Availability|Observe…' ./decoder/
• golangci-lint run — 0 issues

🤖 Generated with Claude Code

[jfberry]

Memory note for the enrichment: the two new FortLookup fields are deliberately int8 placed into existing alignment padding (RaidLevel's pad byte, and the pad before ShowcaseExpiry). Measured with unsafe.Sizeof: FortLookup was 184 bytes before this PR, 192 with naive int16 placement, and 184 again as committed — the enrichment adds zero bytes per resident fort. No DNF filter fields were added; the values feed only the availability index keys (mega X/Y distinctness in pick lists — raid level alone can't distinguish those).

[jfberry]

Added the native-JSON completion for the remaining fort blob columns (stationed_pokemon, showcase_focus, showcase_rankings, both quest_conditions) via jsonRaw — the #385 design said native JSON for blobs but the pass only fully covered gyms. Wire-breaking for those five fields, but both consumers are unmerged: ReactMap's branch already handles both forms, and ccev/diadem#175 now does too (dual-mode blobToString).