firmware: add I2S MEMS microphone driver; fix missing essence migration

Mic: confirmed via its pinout (L/R, WS, SCK, SD, VCC, GND) that the third
target module is a standard I2S digital MEMS mic (INMP441-family). Added
mems_mic.c/h using ESP-IDF's current driver/i2s_std.h API — reports RMS
audio level in dBFS as sensor_type "evp" rather than attempting on-device
voice-band FFT (the browser EVP mode's approach); the backend's existing
statistical anomaly detector handles spike detection from the raw level,
same as it already does for temperature/pressure/presence.

Also fixes a real gap Workstream B's report flagged: User.essence (a live
model column used throughout merged code — /auth/me, inventory purchases,
summon trickle) had no migration line in main.py's lifespan, which would
have broken on the actual production Postgres database.

backend/app/main.py

@@ -54,6 +54,14 @@ async def lifespan(app: FastAPI):
         await conn.execute(text(
             "ALTER TABLE entities ADD COLUMN IF NOT EXISTS traits JSONB NOT NULL DEFAULT '{}'::jsonb"
         ))
+        # Workstream C (character-depth-ghost-log spec) — missing from C's
+        # own commit, added by the integrator after Workstream B's report
+        # flagged that User.essence had a live model column and application
+        # code (auth/me, inventory purchases, summon trickle) but no
+        # migration, which would have broken on the real production DB.
+        await conn.execute(text(
+            "ALTER TABLE users ADD COLUMN IF NOT EXISTS essence INTEGER NOT NULL DEFAULT 0"
+        ))
         # Workstream B (character-depth-ghost-log spec).
         await conn.execute(text(
             "ALTER TABLE users ADD COLUMN IF NOT EXISTS favor DOUBLE PRECISION NOT NULL DEFAULT 0.0"