feat(matrix): add outbound media uploads and unify media limits with maxMediaBytes

- Use OutboundMessage.media for Matrix file/image/audio/video sends
- Apply effective media limit as min(m.upload.size, maxMediaBytes)
- Rename matrix config key maxInboundMediaBytes -> maxMediaBytes (no legacy fallback)

nanobot/config/schema.py

@@ -76,8 +76,8 @@ class MatrixConfig(Base):
     e2ee_enabled: bool = True
     # Max seconds to wait for sync_forever to stop gracefully before cancellation fallback.
     sync_stop_grace_seconds: int = 2
-    # Max attachment size accepted from inbound Matrix media events.
-    max_inbound_media_bytes: int = 20 * 1024 * 1024
+    # Max attachment size accepted for Matrix media handling (inbound + outbound).
+    max_media_bytes: int = 20 * 1024 * 1024
     allow_from: list[str] = Field(default_factory=list)
     group_policy: Literal["open", "mention", "allowlist"] = "open"
     group_allow_from: list[str] = Field(default_factory=list)