docs: add Developer-Patterns page Centralise the canonical backend helpers (utils.key_codec, utils.db_transaction, security.encryption, utils.trusted_proxy, utils.ssrf_protection, utils.response) with rationale tied to past incidents (#103/#104/#105 + latent KeyEncryption misuse + bare commit sites). Each helper section explains what it prevents, when to use it, when NOT to use it, and links to its regression test.

NeySlim committed May 3, 2026