config: accept user_id from file

README.md

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+# Read Later Bot
+
+## Configuration
+
+The bot reads a TOML config passed via `--config`. Most values are standard TOML types. The `user_id` field accepts multiple forms so it can be sourced from secrets managers.
+
+### `user_id`
+
+You can provide the Telegram user ID as:
+
+- A number
+- A numeric string
+- A file path containing the numeric ID (useful for age/sops)
+- An explicit file object
+
+Examples:
+
+```toml
+user_id = 123456789
+```
+
+```toml
+user_id = "123456789"
+```
+
+```toml
+user_id = "/run/agenix/readlater-user-id"
+```
+
+```toml
+user_id = { file = "/run/agenix/readlater-user-id" }
+```