sfm: Event Report PDF generation (v0.20.0 stub layout)

New endpoint GET /db/events/{id}/report.pdf returns a single-page
letter-portrait PDF for any event with waveform data on disk.

Architecture:
  sfm/report_pdf.py — gather_report_data() assembles fields from
    SeismoDb row + .sfm.json sidecar (bw_report block) + .h5 samples;
    render_event_report_pdf() turns that into PDF bytes via matplotlib.
  sfm/server.py — new endpoint wires them together, streams PDF back
    with Content-Disposition: inline so the browser displays it.
  sfm_webapp.html — new "Download PDF" button in the event modal
    footer that opens the endpoint in a new tab.

Fields surfaced — same coverage as a Blastware Event Report:
  Header metadata (date/time, trigger source, range, sample rate,
                   project, client, operator, location, serial+firmware,
                   battery, calibration, file name)
  Microphone block (PSPL in dB(L) + psi, ZC freq, channel test)
  Per-channel stats (PPV, ZC Freq, Time of Peak, Peak Accel,
                     Peak Disp, Sensor Check) for Tran/Vert/Long
  Peak Vector Sum
  Waveform plot (MicL/Long/Vert/Tran stacked, shared time axis,
                 trigger marker, symmetric Y for geo, zero-anchored
                 mic) — OR per-interval bar chart for histograms.

Rendering pipeline = matplotlib only (vector PDF, no headless-browser
dep).  Adds matplotlib>=3.8 to deps.

Visual layout is approximate until reference PDFs from Instantel land
at docs/reference/instantel/ for iteration.  USBM RI8507 / OSMRE
compliance chart is stubbed (placeholder rectangle) — separate work
item.

Smoke-tested on a K558 waveform event: 77 KB valid PDF, all fields
populated correctly from the snapshot DB.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

CHANGELOG.md

@@ -8,6 +8,9 @@ All notable changes to seismo-relay are documented here.
 
 ### Added
 
+- **Event Report PDF generation** — `GET /db/events/{id}/report.pdf` returns a single-page letter-portrait PDF for any event with waveform data on disk.  Covers every field a Blastware Event Report includes: header metadata (date/time, trigger source, range, sample rate, project/client/operator/location, serial+firmware, battery, calibration, file name), microphone block (PSPL in dB(L) + psi, ZC freq, channel test), per-channel stats table (PPV / ZC Freq / Time of Peak / Peak Accel / Peak Disp / Sensor Check), Peak Vector Sum, and the 4-channel waveform plot stacked Instantel-style (MicL top → Tran bottom, shared time axis, trigger marker, symmetric Y on geo channels, zero-anchored on mic).  Histogram events render as per-interval bar charts instead of waveform plots.  USBM RI8507 / OSMRE compliance chart still stubbed — separate work item.  Backed by matplotlib (vector PDF output, no headless-browser dep); new `sfm/report_pdf.py` does data assembly + rendering.  **Visual layout is approximate** until reference PDFs land at `docs/reference/instantel/` to iterate against.
+- **"Download PDF" button** in the event modal's footer — triggers the new endpoint; opens in a new tab so the browser handles save-or-display + surfaces any 404 / server errors visibly.
+
 - **SFM webapp now opens to Database view by default** and the History table is fully interactive.  Click any column header to sort ascending / descending (timestamp, serial, per-channel PPV, PVS, mic dB(L), project, client, record type, key — all sortable).  Click any event row to open the event modal, which now renders a **4-channel waveform plot inline** (MicL / Long / Vert / Tran stacked, Instantel-printout order) alongside the existing sidecar review fields.  Headers are sticky so the columns stay visible while scrolling long event lists.  No more "where is the viewer" — pick a unit from the filter dropdown, scan the table, click the event, see the waveform.
 - **Stored-event browser** — new standalone HTML page at `GET /events` (`sfm/event_browser.html`).  Pick a serial from the unit dropdown, scroll through that unit's events (newest-first), click any event to render its decoded waveform via the existing `/db/events/{id}/waveform.json` endpoint.  Dark-themed Chart.js viewer, channels stacked vertically (MicL / Long / Vert / Tran — Instantel printout order, designed PDF-export-ready), trigger line at t=0, peak labels, search/filter, false-trigger flag honored.  Companion to the existing live-device viewer at `/waveform`; the two routes are now clearly delineated in their docstrings.  The webapp's inline plot at `/` is the primary path; `/events` remains a useful diagnostic when you want just a viewer.
 - **Histogram body codec — uint8 peak count fix.**  Per-channel peak fields at `block[6]/[10]/[14]/[18]` are `uint8`, not `uint16 LE` spanning `block[6:8]` etc.  The original interpretation was byte-exact on the N844 fixture corpus only because every annotation byte (`block[7]/[11]/[15]/[19]`) in those fixtures was zero.  On non-N844 events with non-zero annotation bytes (observed across BE9558 Tran-drift and BE18003 Histogram+Continuous units), the old interpretation produced peaks up to 268 in/s per channel and 35× inflated PVS sums when first deployed to prod (rolled back same day; properly fixed in this release).  Cross-correlated against BW's per-interval ASCII export on K558 / T003 / N599 / N844 corpora — 100% byte-exact on T/V/L, 99%+ on M (sub-precision rounding).  Annotation byte preserved on each record as `record["annotations"]` for future RE.  Verified against ~3,500 blocks across 5 in-repo fixtures + a synthetic K558 interval-12 regression block.