feat(forward): add --skip-same to skip duplicate files - #1341

Proposal

Fixes #1149.

When bulk forwarding from exported JSON files, the same media file may appear multiple times (overlapping exports, cross-posted documents, ...), and every occurrence gets forwarded again. This PR adds a --skip-same flag to tdl forward, mirroring the existing flag in tdl dl.

tdl forward --skip-same -f export.json

Behavior

• A message is skipped if its media has already been forwarded during this run to the same destination peer/thread.
• Media identity is fingerprinted as:
  • document → id + size
  • photo → id
• Dedup is scoped per destination (peer id + thread + media key): routing the same file to two different chats is intentionally not affected.
• Messages without file media are never deduplicated.
• Skipped messages are logged at debug level and counted in a final summary log (skipped duplicated messages count=N).
• Dedup also applies to --dry-run, so the preview matches what a real run would send.
• Docs updated: "Skip Duplicates" section added to EN/ZH forward guide.

Design notes

• Cross-run persistence was considered but intentionally not implemented: unlike dl (where the destination filesystem provides durable truth), forward would need local bookkeeping that can go stale (e.g., destination chat cleaned up), silently dropping legitimate re-forwards. Happy to discuss if maintainers prefer persistent KV-based dedup.
• Grouped messages (albums): dedup applies to the primary message media; album children are not individually fingerprinted (same as upstream grouped handling).

Verification

• go build, go vet, gofmt: clean
• go test -race ./app/forward/...: green (new unit tests for media fingerprinting)
• golangci-lint run app/forward/... cmd/...: 0 issues
• tdl forward --help shows the new flag
• Docs updated (EN + ZH guide)