Add session mode for pre-funded request processing

Implement session-based API endpoints for creating, managing, and interacting with pre-funded sessions, including deposit and top-up invoice generation, macaroon authentication, and per-request debiting of compute costs.

Replit-Commit-Author: Agent
Replit-Commit-Session-Id: 418bf6f8-212b-4bb0-a7a5-8231a061da4e
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Replit-Commit-Event-Id: 2dc3847e-7186-4a22-9c7e-16cd31bca8d9
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lib/db/migrations/0004_sessions.sql

@@ -0,0 +1,39 @@
+-- Migration: Session balance mode (Mode 2)
+-- Users pre-fund a Lightning session; requests auto-debit actual costs.
+
+CREATE TABLE IF NOT EXISTS sessions (
+  id                      TEXT PRIMARY KEY,
+  state                   TEXT NOT NULL DEFAULT 'awaiting_payment',
+  balance_sats            INTEGER NOT NULL DEFAULT 0,
+  deposit_amount_sats     INTEGER NOT NULL,
+  deposit_payment_hash    TEXT NOT NULL,
+  deposit_payment_request TEXT NOT NULL,
+  deposit_paid            BOOLEAN NOT NULL DEFAULT false,
+  topup_amount_sats       INTEGER,
+  topup_payment_hash      TEXT,
+  topup_payment_request   TEXT,
+  topup_paid              BOOLEAN,
+  macaroon                TEXT,
+  expires_at              TIMESTAMPTZ,
+  created_at              TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+  updated_at              TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+
+CREATE TABLE IF NOT EXISTS session_requests (
+  id                TEXT PRIMARY KEY,
+  session_id        TEXT NOT NULL REFERENCES sessions(id),
+  request           TEXT NOT NULL,
+  state             TEXT NOT NULL DEFAULT 'processing',
+  result            TEXT,
+  reason            TEXT,
+  error_message     TEXT,
+  eval_input_tokens  INTEGER,
+  eval_output_tokens INTEGER,
+  work_input_tokens  INTEGER,
+  work_output_tokens INTEGER,
+  debited_sats       INTEGER,
+  balance_after_sats INTEGER,
+  btc_price_usd      REAL,
+  created_at         TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+  updated_at         TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);