feat(v0.6-1): FIRMS handler -- storage-only, closes silent-drop on central.fire.hotspot.>

v0.5.13 default-deny was silently dropping every FIRMS hotspot because no
per-adapter handler existed. firms_pixels table has been empty since v0.5.8b.

This commit adds central/firms_handler.py which stores every passing pixel
that clears the (currently hardcoded, future GUI-driven) confidence + FRP
floors, with dedup on (round(lat,5), round(lon,5), acq_time, satellite) via
a unique partial index added in v4.sql. NO mesh broadcasts emitted by this
handler -- FIRMS data is for LLM context only and will become queryable
when commit #5 (env_reporter) lands.

Defaults baked in:
  FIRMS_CONFIDENCE_FLOOR = "low"   -- store every confidence level
  FIRMS_FRP_FLOOR        = 0.0     -- store every FRP value
  FIRMS_BBOX_OPTIONAL    = None    -- no spatial filter

These become adapter_config GUI rows in commit #3 (per Matt's v0.6 Phase 1
refinement: hardcoded values become GUI default values so first-deploy
behavior is unchanged).

Wiring:
  - meshai/central/firms_handler.py (new, 270 lines)
  - meshai/persistence/migrations/v4.sql (new, unique dedup index)
  - meshai/persistence/db.py (SCHEMA_VERSION 3 -> 4)
  - meshai/central/consumer.py (dispatch ladder gets firms branch before
    default-deny clause; pattern matches handle_swpc / handle_nwis)
  - tests/test_firms_handler.py (new, 22 tests covering confidence floor,
    FRP floor, bbox, dedup, missing fields, end-to-end through consumer)
  - tests/test_consumer_default_deny.py (swap firms -> avalanche for the
    "no handler" examples since FIRMS now has one)

Test count: 658 -> 680 (+22 firms_handler tests, 0 regressions).

Refs audit doc v0.6-phase1-audit.md finding #2.

meshai/persistence/db.py

@@ -30,7 +30,7 @@ logger = logging.getLogger(__name__)
 
 DEFAULT_DB_PATH = "/data/meshai.sqlite"
 MESHAI_DB_PATH_ENV = "MESHAI_DB_PATH"
-SCHEMA_VERSION = 3
+SCHEMA_VERSION = 4
 SCHEMA_META_TABLE = "schema_meta"
 MIGRATIONS_DIR = Path(__file__).parent / "migrations"