feat: merge Feishu/Lark into unified adapter with region toggle

[pluginmd]

Summary

Merge lark.rs into feishu.rs — one unified adapter with region toggle instead of two separate channels.

Why?

Feishu (飞书) and Lark are the same product by ByteDance, just different regions:

	Feishu (CN domestic)	Lark (International)
Domain	open.feishu.cn	open.larksuite.com
Market	China mainland	Global (SEA, JP, US, EU…)
API	Identical endpoints & schemas	Identical endpoints & schemas
Auth	Same tenant_access_token flow	Same tenant_access_token flow
Events	Same V2 webhook callback format	Same V2 webhook callback format

The only difference is the base URL domain. Maintaining two separate adapters (feishu.rs + lark.rs) was redundant — same code, different domain string.

What changed

Single [channels.feishu] config with region field ("cn" or "intl") controls which API domain is used:

• region = "cn" → open.feishu.cn (Feishu domestic, default)
• region = "intl" → open.larksuite.com (Lark international)

Features merged from lark.rs into feishu.rs

• Event decryption — AES-256-CBC decrypt for encrypted webhook payloads
• Message dedup — Ring-buffer cache preventing duplicate message processing
• Event dedup — Separate ring-buffer for event_id deduplication
• Group chat filtering — Only respond when @mentioned, question mark, or bot name detected
• Rich text parsing — Extract text from post (rich text) messages with locale fallback
• @mention stripping — Clean @_user_1 placeholders from message text

Changes

File	What
crates/openfang-channels/src/feishu.rs	Complete rewrite — merged all lark.rs features, added FeishuRegion enum with domain switching
crates/openfang-types/src/config.rs	Expanded FeishuConfig: +5 fields (region, webhook_path, verification_token, encrypt_key_env, bot_names)
crates/openfang-api/src/channel_bridge.rs	Updated wiring to pass all new config fields to FeishuAdapter::with_config()
crates/openfang-api/src/routes.rs	Updated ChannelMeta UI — region as basic field, 6 new advanced fields

Config example

[channels.feishu]
app_id = "cli_abc123"
app_secret_env = "FEISHU_APP_SECRET"
region = "cn"                          # "cn" = Feishu, "intl" = Lark
# webhook_port = 8453
# webhook_path = "/feishu/webhook"
# verification_token = "your-verify-token"
# encrypt_key_env = "FEISHU_ENCRYPT_KEY"
# bot_names = ["MyBot", "Assistant"]

Test plan

• cargo build --workspace --lib — compiles
• cargo test --workspace — all tests pass
• cargo clippy -p openfang-channels -p openfang-api -p openfang-types -- -D warnings — zero warnings
• Live server: /api/channels returns all 9 Feishu fields correctly
• Manual: configure with real Feishu CN credentials
• Manual: configure with real Lark International credentials

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