feat(srt): add SRT statistics for inputs and outputs

[srperens]

Summary

Adds on-demand SRT statistics for all four SRT block types (efpsrt_input, efpsrt_output, mpegtssrt_input, mpegtssrt_output), mirroring the existing WebRTC stats pattern.

• Backend scans the pipeline for srtsrc / srtsink elements and reads their libsrt stats GstStructure. Handles caller-mode top-level, listener-mode GValueArray, and gst::Array / gst::List variants across gst-plugins-bad versions. Exposed via GET /api/flows/{id}/srt-stats.
• API uses typed SrtRole / SrtMode enums and splits sender vs receiver counters (packets_sent_lost / packets_received_lost etc.) so the semantics of each counter is unambiguous. Peer address parsed from GInetSocketAddress and formatted as ip:port.
• Frontend polls every 1s when the selected flow has an SRT block. Compact inline view in the graph node shows peer count, RTT, and recent loss %. The property-inspector view renders per-caller sections, separating NAK'd-then-retransmitted (recovered, green) from TLPKTDROP / TSBPD (actual data loss, red).
• Per-second deltas are computed against the previous poll so loss / packet / byte counters show both lifetime totals and current rate (134k (450/s)), keeping the figures meaningful on long-running streams. Recent-interval loss fraction drives both the compact loss reading and a per-caller "Recent loss" row.
• 8 unit tests cover the parser (sender / receiver split, listener callers array, address parsing, integer-width aliases, connected-flag heuristics).

Test plan

• cargo check passes on backend + frontend (native + WASM)
• cargo test --lib pipeline::srt — 8/8 pass
• cargo test --test openapi_test — snapshot updated and clean
• Pre-commit hooks (fmt, clippy, sensitive-content) green
• Manual: caller-mode srtsrc receives stream → stats appear in inspector with correct receiver-side fields
• Manual: listener-mode srtsink with 2 connected peers → both peers visible with addresses, per-caller stats, recent-loss readout
• Manual: confirmed sender-side NAK'd-then-recovered loss displays green (Retransmitted: 678 (100% recovered)) rather than red, while TLPKTDROP would display red

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