Add email channel via Mailgun for Ask Simba

Users can now receive a unique email address (ask+<token>@domain) and
interact with Simba by sending emails. Inbound emails hit a Mailgun
webhook, are authenticated via HMAC token lookup, processed through the
LangChain agent, and replied to via the Mailgun API.

- Extract shared SIMBA_SYSTEM_PROMPT to blueprints/conversation/prompts.py
- Add email_enabled and email_hmac_token fields to User model
- Create blueprints/email with webhook, signature validation, rate limiting
- Add admin endpoints to enable/disable email per user
- Update AdminPanel with Email column, toggle, and copy-address button
- Add Mailgun env vars to .env.example
- Include database migration for new fields

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

migrations/models/3_20260313000000_add_email_fields.py

@@ -0,0 +1,17 @@
+from tortoise import BaseDBAsyncClient
+
+RUN_IN_TRANSACTION = True
+
+
+async def upgrade(db: BaseDBAsyncClient) -> str:
+    return """
+        ALTER TABLE "users" ADD "email_enabled" BOOL NOT NULL DEFAULT FALSE;
+        ALTER TABLE "users" ADD "email_hmac_token" VARCHAR(16) UNIQUE;
+        CREATE INDEX "idx_users_email_h_a1b2c3" ON "users" ("email_hmac_token");"""
+
+
+async def downgrade(db: BaseDBAsyncClient) -> str:
+    return """
+        DROP INDEX IF EXISTS "idx_users_email_h_a1b2c3";
+        ALTER TABLE "users" DROP COLUMN "email_hmac_token";
+        ALTER TABLE "users" DROP COLUMN "email_enabled";"""